CFG.txt

<program>	->	<dec><program>|^
<dec>		->	<type>ident<dectail>|semicon|rsv_extern<type>ident semicon
<type>		->	rsv_void|rsv_int|rsv_char
<dectail>	->	semicon|<varlist>semicon|lparen<para>rparen<funtail>
<funtail>	->	<block>|semicon
<varlist>	->	comma ident<varlist>|^
<para>		->	<type>ident<paralist>|^
<paralist>	->	comma<type>ident<paralist>|^

<block>		->	lbrac<childprogram>rbrac
<childprogram>	->	<localdec><childprogram>|<statements><childprogram>|^
<localdec>	->	<type>ident<lcoaldectail>semicon
<localvartail>	->	comma ident<localvartail>|^

<statement>	->	ident<idtail>semicon|<whilestat>|<ifstat>|<retstat>|semicon
			|rsv_break semicon|rsv_continue semicon
			|rsv_in input<ident>semicon|rsv_out output<expr>semicon
<whilestat>	->	rsv_while lparen<expr>rparen<block>
<ifstat>	->	rsv_if lparen<expr>rparen<block>rsv_else<block>
<retstat>	->	rsv_return<returntail>semicon
<returntail>	->	<expr>|^
<idtail>	->	assign<exp>|lparen<realarg>rparen
<realarg>	->	<expr><arglist>|^
<arglist>	->	comma<expr><arglist>|^

<expr>		->	<aloexp><exptail>
<exptail>	->	<cmps><expr>|^
<cmps>		->	gt|ge|ls|le|equ|nequ
<aloexp>	->	<item><itemtail>
<itemtail>	->	<adds><aloexp>|^
<adds>		->	add|sub
<item>		->	<factor><factortail>
<factortail>	->	<muls><item>|^
<factor>	->	ident<idtail>|number|chara|lparen<expr>rparen|string
<muls>		->	mul|div