CHANGES

Version 2.4
-----------------------------
04/28/08: beazley
          Fixed a big with p_erro() functions being picked up correctly
          when running in yacc(optimize=1) mode.  Patch contributed by
          Bart Whiteley.

02/28/08: beazley
          Fixed a bug with 'nonassoc' precedence rules.   Basically the
          non-precedence was being ignored and not producing the correct
          run-time behavior in the parser.

02/16/08: beazley
          Slight relaxation of what the input() method to a lexer will
          accept as a string.   Instead of testing the input to see
          if the input is a string or unicode string, it checks to see
          if the input object looks like it contains string data.
          This change makes it possible to pass string-like objects
          in as input.  For example, the object returned by mmap.

              import mmap, os
              data = mmap.mmap(os.open(filename,os.O_RDONLY),
                               os.path.getsize(filename),
                               access=mmap.ACCESS_READ)
              lexer.input(data)
           

11/29/07: beazley
          Modification of ply.lex to allow token functions to aliased.
          This is subtle, but it makes it easier to create libraries and
          to reuse token specifications.  For example, suppose you defined
          a function like this:

               def number(t):
                    r'\d+'
                    t.value = int(t.value)
                    return t

          This change would allow you to define a token rule as follows:

              t_NUMBER = number

          In this case, the token type will be set to 'NUMBER' and use
          the associated number() function to process tokens.   
           
11/28/07: beazley
          Slight modification to lex and yacc to grab symbols from both
          the local and global dictionaries of the caller.   This
          modification allows lexers and parsers to be defined using
          inner functions and closures.

11/28/07: beazley
          Performance optimization:  The lexer.lexmatch and t.lexer
          attributes are no longer set for lexer tokens that are not
          defined by functions.   The only normal use of these attributes
          would be in lexer rules that need to perform some kind of 
          special processing.  Thus, it doesn't make any sense to set
          them on every token.

          *** POTENTIAL INCOMPATIBILITY ***  This might break code
          that is mucking around with internal lexer state in some
          sort of magical way.

11/27/07: beazley
          Added the ability to put the parser into error-handling mode
          from within a normal production.   To do this, simply call
          the .error() method like this:

          def p_some_production(p):
              'some_production : prod1 prod2'
              ...
              p.error()      # Signal an error

          A number of things happen after error() has been called.

          - The last symbol shifted onto the symbol stack is discarded
            and parser state backed up to what it was before the
            the rule reduction.

          - The current lookahead symbol is saved and replaced by
            the 'error' symbol.

          - The parser enters error recovery mode where it tries
            to either reduce the 'error' rule or it starts 
            discarding items off of the stack until the parser
            resets.

          When an error is manually set, the parser does *not* call 
          the p_error() function (if any is defined).
          *** NEW FEATURE *** Suggested on the mailing list

11/27/07: beazley
          Fixed structure bug in examples/ansic.  Reported by Dion Blazakis.

11/27/07: beazley
          Fixed a bug in the lexer related to start conditions and ignored
          token rules.  If a rule was defined that changed state, but
          returned no token, the lexer could be left in an inconsistent
          state.  Reported by 
          
11/27/07: beazley
          Modified setup.py to support Python Eggs.   Patch contributed by
          Simon Cross.

11/09/07: beazely
          Fixed a bug in error handling in yacc.  If a syntax error occurred and the
          parser rolled the entire parse stack back, the parser would be left in in
          inconsistent state that would cause it to trigger incorrect actions on
          subsequent input.  Reported by Ton Biegstraaten, Justin King, and others.

11/09/07: beazley
          Fixed a bug when passing empty input strings to yacc.parse().   This 
          would result in an error message about "No input given".  Reported
          by Andrew Dalke.

03/13/07: beazley
          Modified the LexToken objects returned by the lexer so that they support
          comparisons.  Comparisons can be performed against simple strings like
          this:

               if tok == 'NUMBER':
                  ...

          In this case, the token type field is compared against the string. You
          can also compare against a tuple, like this:

              if tok == ('ID','foo')

          In this case, the tuple is taken to be the token type and token value.
          Last, but not least, two different tokens can be compared.  Tokens are
          equal if they have the same type and value.

          One useful aspect of this addition is that token sequences can be easily
          compared.  For example:

              if toks == ['NUMBER','+','NUMBER']:

Version 2.3
-----------------------------
02/20/07: beazley
          Fixed a bug with character literals if the literal '.' appeared as the
          last symbol of a grammar rule.  Reported by Ales Smrcka.

02/19/07: beazley
          Warning messages are now redirected to stderr instead of being printed
          to standard output.

02/19/07: beazley
          Added a warning message to lex.py if it detects a literal backslash
          character inside the t_ignore declaration.  This is to help
          problems that might occur if someone accidentally defines t_ignore
          as a Python raw string.  For example:

              t_ignore = r' \t'

          The idea for this is from an email I received from David Cimimi who
          reported bizarre behavior in lexing as a result of defining t_ignore
          as a raw string by accident.

02/18/07: beazley
          Performance improvements.  Made some changes to the internal
          table organization and LR parser to improve parsing performance.

02/18/07: beazley
          Automatic tracking of line number and position information must now be 
          enabled by a special flag to parse().  For example:

              yacc.parse(data,tracking=True)

          In many applications, it's just not that important to have the
          parser automatically track all line numbers.  By making this an 
          optional feature, it allows the parser to run significantly faster
          (more than a 20% speed increase in many cases).    Note: positional
          information is always available for raw tokens---this change only
          applies to positional information associated with nonterminal
          grammar symbols.
          *** POTENTIAL INCOMPATIBILITY ***
	  
02/18/07: beazley
          Yacc no longer supports extended slices of grammar productions.
          However, it does support regular slices.  For example:

          def p_foo(p):
              '''foo: a b c d e'''
              p[0] = p[1:3]

          This change is a performance improvement to the parser--it streamlines
          normal access to the grammar values since slices are now handled in
          a __getslice__() method as opposed to __getitem__().

02/12/07: beazley
          Fixed a bug in the handling of token names when combined with
          start conditions.   Bug reported by Todd O'Bryan.

Version 2.2
------------------------------
11/01/06: beazley
          Added lexpos() and lexspan() methods to grammar symbols.  These
          mirror the same functionality of lineno() and linespan().  For
          example:

          def p_expr(p):
              'expr : expr PLUS expr'
               p.lexpos(1)     # Lexing position of left-hand-expression
               p.lexpos(1)     # Lexing position of PLUS
               start,end = p.lexspan(3)  # Lexing range of right hand expression

11/01/06: beazley
          Minor change to error handling.  The recommended way to skip characters
          in the input is to use t.lexer.skip() as shown here:

             def t_error(t):
                 print "Illegal character '%s'" % t.value[0]
                 t.lexer.skip(1)
          
          The old approach of just using t.skip(1) will still work, but won't
          be documented.

10/31/06: beazley
          Discarded tokens can now be specified as simple strings instead of
          functions.  To do this, simply include the text "ignore_" in the
          token declaration.  For example:

              t_ignore_cppcomment = r'//.*'
          
          Previously, this had to be done with a function.  For example:

              def t_ignore_cppcomment(t):
                  r'//.*'
                  pass

          If start conditions/states are being used, state names should appear
          before the "ignore_" text.

10/19/06: beazley
          The Lex module now provides support for flex-style start conditions
          as described at http://www.gnu.org/software/flex/manual/html_chapter/flex_11.html.
          Please refer to this document to understand this change note.  Refer to
          the PLY documentation for PLY-specific explanation of how this works.

          To use start conditions, you first need to declare a set of states in
          your lexer file:

          states = (
                    ('foo','exclusive'),
                    ('bar','inclusive')
          )

          This serves the same role as the %s and %x specifiers in flex.

          One a state has been declared, tokens for that state can be 
          declared by defining rules of the form t_state_TOK.  For example:

            t_PLUS = '\+'          # Rule defined in INITIAL state
            t_foo_NUM = '\d+'      # Rule defined in foo state
            t_bar_NUM = '\d+'      # Rule defined in bar state

            t_foo_bar_NUM = '\d+'  # Rule defined in both foo and bar
            t_ANY_NUM = '\d+'      # Rule defined in all states

          In addition to defining tokens for each state, the t_ignore and t_error
          specifications can be customized for specific states.  For example:

            t_foo_ignore = " "     # Ignored characters for foo state
            def t_bar_error(t):   
                # Handle errors in bar state

          With token rules, the following methods can be used to change states
          
            def t_TOKNAME(t):
                t.lexer.begin('foo')        # Begin state 'foo'
                t.lexer.push_state('foo')   # Begin state 'foo', push old state
                                            # onto a stack
                t.lexer.pop_state()         # Restore previous state
                t.lexer.current_state()     # Returns name of current state

          These methods mirror the BEGIN(), yy_push_state(), yy_pop_state(), and
          yy_top_state() functions in flex.

          The use of start states can be used as one way to write sub-lexers.
          For example, the lexer or parser might instruct the lexer to start
          generating a different set of tokens depending on the context.
          
          example/yply/ylex.py shows the use of start states to grab C/C++ 
          code fragments out of traditional yacc specification files.

          *** NEW FEATURE *** Suggested by Daniel Larraz with whom I also
          discussed various aspects of the design.

10/19/06: beazley
          Minor change to the way in which yacc.py was reporting shift/reduce
          conflicts.  Although the underlying LALR(1) algorithm was correct,
          PLY was under-reporting the number of conflicts compared to yacc/bison
          when precedence rules were in effect.  This change should make PLY
          report the same number of conflicts as yacc.

10/19/06: beazley
          Modified yacc so that grammar rules could also include the '-' 
          character.  For example:

            def p_expr_list(p):
                'expression-list : expression-list expression'

          Suggested by Oldrich Jedlicka.

10/18/06: beazley
          Attribute lexer.lexmatch added so that token rules can access the re 
          match object that was generated.  For example:

          def t_FOO(t):
              r'some regex'
              m = t.lexer.lexmatch
              # Do something with m


          This may be useful if you want to access named groups specified within
          the regex for a specific token. Suggested by Oldrich Jedlicka.
          
10/16/06: beazley
          Changed the error message that results if an illegal character
          is encountered and no default error function is defined in lex.
          The exception is now more informative about the actual cause of
          the error.
      
Version 2.1
------------------------------
10/02/06: beazley
          The last Lexer object built by lex() can be found in lex.lexer.
          The last Parser object built  by yacc() can be found in yacc.parser.

10/02/06: beazley
          New example added:  examples/yply

          This example uses PLY to convert Unix-yacc specification files to
          PLY programs with the same grammar.   This may be useful if you
          want to convert a grammar from bison/yacc to use with PLY.
    
10/02/06: beazley
          Added support for a start symbol to be specified in the yacc
          input file itself.  Just do this:

               start = 'name'

          where 'name' matches some grammar rule.  For example:

               def p_name(p):
                   'name : A B C'
                   ...

          This mirrors the functionality of the yacc %start specifier.

09/30/06: beazley
          Some new examples added.:

          examples/GardenSnake : A simple indentation based language similar
                                 to Python.  Shows how you might handle 
                                 whitespace.  Contributed by Andrew Dalke.

          examples/BASIC       : An implementation of 1964 Dartmouth BASIC.
                                 Contributed by Dave against his better
                                 judgement.

09/28/06: beazley
          Minor patch to allow named groups to be used in lex regular
          expression rules.  For example:

              t_QSTRING = r'''(?P<quote>['"]).*?(?P=quote)'''

          Patch submitted by Adam Ring.
 
09/28/06: beazley
          LALR(1) is now the default parsing method.   To use SLR, use
          yacc.yacc(method="SLR").  Note: there is no performance impact
          on parsing when using LALR(1) instead of SLR. However, constructing
          the parsing tables will take a little longer.

09/26/06: beazley
          Change to line number tracking.  To modify line numbers, modify
          the line number of the lexer itself.  For example:

          def t_NEWLINE(t):
              r'\n'
              t.lexer.lineno += 1

          This modification is both cleanup and a performance optimization.
          In past versions, lex was monitoring every token for changes in
          the line number.  This extra processing is unnecessary for a vast
          majority of tokens. Thus, this new approach cleans it up a bit.

          *** POTENTIAL INCOMPATIBILITY ***
          You will need to change code in your lexer that updates the line
          number. For example, "t.lineno += 1" becomes "t.lexer.lineno += 1"
         
09/26/06: beazley
          Added the lexing position to tokens as an attribute lexpos. This
          is the raw index into the input text at which a token appears.
          This information can be used to compute column numbers and other
          details (e.g., scan backwards from lexpos to the first newline
          to get a column position).

09/25/06: beazley
          Changed the name of the __copy__() method on the Lexer class
          to clone().  This is used to clone a Lexer object (e.g., if
          you're running different lexers at the same time).

09/21/06: beazley
          Limitations related to the use of the re module have been eliminated.
          Several users reported problems with regular expressions exceeding
          more than 100 named groups. To solve this, lex.py is now capable
          of automatically splitting its master regular regular expression into
          smaller expressions as needed.   This should, in theory, make it
          possible to specify an arbitrarily large number of tokens.

09/21/06: beazley
          Improved error checking in lex.py.  Rules that match the empty string
          are now rejected (otherwise they cause the lexer to enter an infinite
          loop).  An extra check for rules containing '#' has also been added.
          Since lex compiles regular expressions in verbose mode, '#' is interpreted
          as a regex comment, it is critical to use '\#' instead.  

09/18/06: beazley
          Added a @TOKEN decorator function to lex.py that can be used to 
          define token rules where the documentation string might be computed
          in some way.
          
          digit            = r'([0-9])'
          nondigit         = r'([_A-Za-z])'
          identifier       = r'(' + nondigit + r'(' + digit + r'|' + nondigit + r')*)'        

          from ply.lex import TOKEN

          @TOKEN(identifier)
          def t_ID(t):
               # Do whatever

          The @TOKEN decorator merely sets the documentation string of the
          associated token function as needed for lex to work.  

          Note: An alternative solution is the following:

          def t_ID(t):
              # Do whatever
   
          t_ID.__doc__ = identifier

          Note: Decorators require the use of Python 2.4 or later.  If compatibility
          with old versions is needed, use the latter solution.

          The need for this feature was suggested by Cem Karan.

09/14/06: beazley
          Support for single-character literal tokens has been added to yacc.
          These literals must be enclosed in quotes.  For example:

          def p_expr(p):
               "expr : expr '+' expr"
               ...

          def p_expr(p):
               'expr : expr "-" expr'
               ...

          In addition to this, it is necessary to tell the lexer module about
          literal characters.   This is done by defining the variable 'literals'
          as a list of characters.  This should  be defined in the module that
          invokes the lex.lex() function.  For example:

             literals = ['+','-','*','/','(',')','=']
 
          or simply

             literals = '+=*/()='

          It is important to note that literals can only be a single character.
          When the lexer fails to match a token using its normal regular expression
          rules, it will check the current character against the literal list.
          If found, it will be returned with a token type set to match the literal
          character.  Otherwise, an illegal character will be signalled.


09/14/06: beazley
          Modified PLY to install itself as a proper Python package called 'ply'.
          This will make it a little more friendly to other modules.  This
          changes the usage of PLY only slightly.  Just do this to import the
          modules

                import ply.lex as lex
                import ply.yacc as yacc

          Alternatively, you can do this:

                from ply import *

          Which imports both the lex and yacc modules.
          Change suggested by Lee June.

09/13/06: beazley
          Changed the handling of negative indices when used in production rules.
          A negative production index now accesses already parsed symbols on the
          parsing stack.  For example, 

              def p_foo(p):
                   "foo: A B C D"
                   print p[1]       # Value of 'A' symbol
                   print p[2]       # Value of 'B' symbol
                   print p[-1]      # Value of whatever symbol appears before A
                                    # on the parsing stack.

                   p[0] = some_val  # Sets the value of the 'foo' grammer symbol
                                    
          This behavior makes it easier to work with embedded actions within the
          parsing rules. For example, in C-yacc, it is possible to write code like
          this:

               bar:   A { printf("seen an A = %d\n", $1); } B { do_stuff; }

          In this example, the printf() code executes immediately after A has been
          parsed.  Within the embedded action code, $1 refers to the A symbol on
          the stack.

          To perform this equivalent action in PLY, you need to write a pair
          of rules like this:

               def p_bar(p):
                     "bar : A seen_A B"
                     do_stuff

               def p_seen_A(p):
                     "seen_A :"
                     print "seen an A =", p[-1]

          The second rule "seen_A" is merely a empty production which should be
          reduced as soon as A is parsed in the "bar" rule above.  The use 
          of the negative index p[-1] is used to access whatever symbol appeared
          before the seen_A symbol.

          This feature also makes it possible to support inherited attributes.
          For example:

               def p_decl(p):
                     "decl : scope name"

               def p_scope(p):
                     """scope : GLOBAL
                              | LOCAL"""
                   p[0] = p[1]

               def p_name(p):
                     "name : ID"
                     if p[-1] == "GLOBAL":
                          # ...
                     else if p[-1] == "LOCAL":
                          #...

          In this case, the name rule is inheriting an attribute from the
          scope declaration that precedes it.
       
          *** POTENTIAL INCOMPATIBILITY ***
          If you are currently using negative indices within existing grammar rules,
          your code will break.  This should be extremely rare if non-existent in
          most cases.  The argument to various grammar rules is not usually not
          processed in the same way as a list of items.
          
Version 2.0
------------------------------
09/07/06: beazley
          Major cleanup and refactoring of the LR table generation code.  Both SLR
          and LALR(1) table generation is now performed by the same code base with
          only minor extensions for extra LALR(1) processing.

09/07/06: beazley
          Completely reimplemented the entire LALR(1) parsing engine to use the
          DeRemer and Pennello algorithm for calculating lookahead sets.  This
          significantly improves the performance of generating LALR(1) tables
          and has the added feature of actually working correctly!  If you
          experienced weird behavior with LALR(1) in prior releases, this should
          hopefully resolve all of those problems.  Many thanks to 
          Andrew Waters and Markus Schoepflin for submitting bug reports
          and helping me test out the revised LALR(1) support.

Version 1.8
------------------------------
08/02/06: beazley
          Fixed a problem related to the handling of default actions in LALR(1)
          parsing.  If you experienced subtle and/or bizarre behavior when trying
          to use the LALR(1) engine, this may correct those problems.  Patch
          contributed by Russ Cox.  Note: This patch has been superceded by 
          revisions for LALR(1) parsing in Ply-2.0.

08/02/06: beazley
          Added support for slicing of productions in yacc.  
          Patch contributed by Patrick Mezard.

Version 1.7
------------------------------
03/02/06: beazley
          Fixed infinite recursion problem ReduceToTerminals() function that
          would sometimes come up in LALR(1) table generation.  Reported by 
          Markus Schoepflin.

03/01/06: beazley
          Added "reflags" argument to lex().  For example:

               lex.lex(reflags=re.UNICODE)

          This can be used to specify optional flags to the re.compile() function
          used inside the lexer.   This may be necessary for special situations such
          as processing Unicode (e.g., if you want escapes like \w and \b to consult
          the Unicode character property database).   The need for this suggested by
          Andreas Jung.

03/01/06: beazley
          Fixed a bug with an uninitialized variable on repeated instantiations of parser
          objects when the write_tables=0 argument was used.   Reported by Michael Brown.

03/01/06: beazley
          Modified lex.py to accept Unicode strings both as the regular expressions for
          tokens and as input. Hopefully this is the only change needed for Unicode support.
          Patch contributed by Johan Dahl.

03/01/06: beazley
          Modified the class-based interface to work with new-style or old-style classes.
          Patch contributed by Michael Brown (although I tweaked it slightly so it would work
          with older versions of Python).

Version 1.6
------------------------------
05/27/05: beazley
          Incorporated patch contributed by Christopher Stawarz to fix an extremely
          devious bug in LALR(1) parser generation.   This patch should fix problems
          numerous people reported with LALR parsing.

05/27/05: beazley
          Fixed problem with lex.py copy constructor.  Reported by Dave Aitel, Aaron Lav,
          and Thad Austin. 

05/27/05: beazley
          Added outputdir option to yacc()  to control output directory. Contributed
          by Christopher Stawarz.

05/27/05: beazley
          Added rununit.py test script to run tests using the Python unittest module.
          Contributed by Miki Tebeka.

Version 1.5
------------------------------
05/26/04: beazley
          Major enhancement. LALR(1) parsing support is now working.
          This feature was implemented by Elias Ioup (ezioup@alumni.uchicago.edu)
          and optimized by David Beazley. To use LALR(1) parsing do
          the following:

               yacc.yacc(method="LALR")

          Computing LALR(1) parsing tables takes about twice as long as
          the default SLR method.  However, LALR(1) allows you to handle
          more complex grammars.  For example, the ANSI C grammar
          (in example/ansic) has 13 shift-reduce conflicts with SLR, but
          only has 1 shift-reduce conflict with LALR(1).

05/20/04: beazley
          Added a __len__ method to parser production lists.  Can
          be used in parser rules like this:

             def p_somerule(p):
                 """a : B C D
                      | E F"
                 if (len(p) == 3):
                     # Must have been first rule
                 elif (len(p) == 2):
                     # Must be second rule

          Suggested by Joshua Gerth and others.

Version 1.4
------------------------------
04/23/04: beazley
          Incorporated a variety of patches contributed by Eric Raymond.
          These include:

           0. Cleans up some comments so they don't wrap on an 80-column display.
           1. Directs compiler errors to stderr where they belong.
           2. Implements and documents automatic line counting when \n is ignored.
           3. Changes the way progress messages are dumped when debugging is on. 
              The new format is both less verbose and conveys more information than
              the old, including shift and reduce actions.

04/23/04: beazley
          Added a Python setup.py file to simply installation.  Contributed
          by Adam Kerrison.

04/23/04: beazley
          Added patches contributed by Adam Kerrison.
 
          -   Some output is now only shown when debugging is enabled.  This
              means that PLY will be completely silent when not in debugging mode.
          
          -   An optional parameter "write_tables" can be passed to yacc() to
              control whether or not parsing tables are written.   By default,
              it is true, but it can be turned off if you don't want the yacc
              table file. Note: disabling this will cause yacc() to regenerate
              the parsing table each time.

04/23/04: beazley
          Added patches contributed by David McNab.  This patch addes two
          features:

          -   The parser can be supplied as a class instead of a module.
              For an example of this, see the example/classcalc directory.

          -   Debugging output can be directed to a filename of the user's
              choice.  Use

                 yacc(debugfile="somefile.out")

          
Version 1.3
------------------------------
12/10/02: jmdyck
          Various minor adjustments to the code that Dave checked in today.
          Updated test/yacc_{inf,unused}.exp to reflect today's changes.

12/10/02: beazley
          Incorporated a variety of minor bug fixes to empty production
          handling and infinite recursion checking.  Contributed by
          Michael Dyck.

12/10/02: beazley
          Removed bogus recover() method call in yacc.restart()

Version 1.2
------------------------------
11/27/02: beazley
          Lexer and parser objects are now available as an attribute
          of tokens and slices respectively. For example:
 
             def t_NUMBER(t):
                 r'\d+'
                 print t.lexer

             def p_expr_plus(t):
                 'expr: expr PLUS expr'
                 print t.lexer
                 print t.parser

          This can be used for state management (if needed).
 
10/31/02: beazley
          Modified yacc.py to work with Python optimize mode.  To make
          this work, you need to use

              yacc.yacc(optimize=1)

          Furthermore, you need to first run Python in normal mode
          to generate the necessary parsetab.py files.  After that,
          you can use python -O or python -OO.  

          Note: optimized mode turns off a lot of error checking.
          Only use when you are sure that your grammar is working.
          Make sure parsetab.py is up to date!

10/30/02: beazley
          Added cloning of Lexer objects.   For example:

              import copy
              l = lex.lex()
              lc = copy.copy(l)

              l.input("Some text")
              lc.input("Some other text")
              ...

          This might be useful if the same "lexer" is meant to
          be used in different contexts---or if multiple lexers
          are running concurrently.
                
10/30/02: beazley
          Fixed subtle bug with first set computation and empty productions.
          Patch submitted by Michael Dyck.

10/30/02: beazley
          Fixed error messages to use "filename:line: message" instead
          of "filename:line. message".  This makes error reporting more
          friendly to emacs. Patch submitted by François Pinard.

10/30/02: beazley
          Improvements to parser.out file.  Terminals and nonterminals
          are sorted instead of being printed in random order.
          Patch submitted by François Pinard.

10/30/02: beazley
          Improvements to parser.out file output.  Rules are now printed
          in a way that's easier to understand.  Contributed by Russ Cox.

10/30/02: beazley
          Added 'nonassoc' associativity support.    This can be used
          to disable the chaining of operators like a < b < c.
          To use, simply specify 'nonassoc' in the precedence table

          precedence = (
            ('nonassoc', 'LESSTHAN', 'GREATERTHAN'),  # Nonassociative operators
            ('left', 'PLUS', 'MINUS'),
            ('left', 'TIMES', 'DIVIDE'),
            ('right', 'UMINUS'),            # Unary minus operator
          )

          Patch contributed by Russ Cox.

10/30/02: beazley
          Modified the lexer to provide optional support for Python -O and -OO
          modes.  To make this work, Python *first* needs to be run in
          unoptimized mode.  This reads the lexing information and creates a
          file "lextab.py".  Then, run lex like this:

                   # module foo.py
                   ...
                   ...
                   lex.lex(optimize=1)

          Once the lextab file has been created, subsequent calls to
          lex.lex() will read data from the lextab file instead of using 
          introspection.   In optimized mode (-O, -OO) everything should
          work normally despite the loss of doc strings.

          To change the name of the file 'lextab.py' use the following:

                  lex.lex(lextab="footab")

          (this creates a file footab.py)
         

Version 1.1   October 25, 2001
------------------------------

10/25/01: beazley
          Modified the table generator to produce much more compact data.
          This should greatly reduce the size of the parsetab.py[c] file.
          Caveat: the tables still need to be constructed so a little more
          work is done in parsetab on import. 

10/25/01: beazley
          There may be a possible bug in the cycle detector that reports errors
          about infinite recursion.   I'm having a little trouble tracking it
          down, but if you get this problem, you can disable the cycle
          detector as follows:

                 yacc.yacc(check_recursion = 0)

10/25/01: beazley
          Fixed a bug in lex.py that sometimes caused illegal characters to be
          reported incorrectly.  Reported by Sverre Jørgensen.

7/8/01  : beazley
          Added a reference to the underlying lexer object when tokens are handled by
          functions.   The lexer is available as the 'lexer' attribute.   This
          was added to provide better lexing support for languages such as Fortran
          where certain types of tokens can't be conveniently expressed as regular 
          expressions (and where the tokenizing function may want to perform a 
          little backtracking).  Suggested by Pearu Peterson.

6/20/01 : beazley
          Modified yacc() function so that an optional starting symbol can be specified.
          For example:
            
                 yacc.yacc(start="statement")

          Normally yacc always treats the first production rule as the starting symbol.
          However, if you are debugging your grammar it may be useful to specify
          an alternative starting symbol.  Idea suggested by Rich Salz.
                      
Version 1.0  June 18, 2001
--------------------------
Initial public offering