Symbol drawing commands are stored in a text file (one file for each symbol)
which are located in $GISBASE/etc/symbol/group/name or mapset/symbol/group/name

S_read() searches first in mapsets and then in $GISBASE (symbol names in 
$GISBASE may be overridden by symbols of the same group/name in mapset).


File format example: 
VERSION 1.0
BOX -10 -10 10 10
STRING
  LINE
    0 -10 
    0  10
  END
END

A symbol file is composed of objects, each object starts with a keyword and
ends with a new line or 'END'. Some objects may contain other objects.

- Drawing is composed of POLYGONS and STRINGS.
- POLYGONS are composed of RINGS. The first ring is outer (clockwise for now),
  following rings are inner (counter clockwise currently).
- STRINGS and RINGS are composed of LINES and ARCS.

POLYGONS and STRINGS are drawn using the default color and fill color
(specified in module options such as 'd.vect color='). This color may be 
overwritten by using the COLOR and FCOLOR keywords.

Blank lines and lines starting with a #hash will be ignored.

---- DESCRIPTION OF OBJECTS ----

In the following descriptions these variables are used:
  N     - integer number
  R G B - RGB color as 3 numbers ranging 0-255
  X,Y   - coordinates (double precision floating point)


Keywords:

VERSION N.N
 Version number

BOX X Y X Y
 Bounding box. Lower left and upper right corner. Larger side of box is taken
 as 1 for symbol scaling. That means that if size of box is 10 x 5, and scale
 in application is 2, all geometry will be multiplied by: 2 / 10.
 Origin of symbol is 0,0.

COLOR NONE | R G B
 Color used for string or outline of polygon. If NONE is used, then the polygon
 outline is not drawn.

FCOLOR NONE | R G B
 Fill color used for polygon. If NONE is used, then the polygon is not filled.

LINE 
  X Y
  X Y
 [X Y]
END
 Line constructed from coordinate pairs.

ARC X Y R A1 A2 [C]
 Arc specified by center (X Y), radius (R), start/end angle in degrees (A1/A2)
 and optionally direction (C for clockwise, default counter clockwise)
 Note degrees are polar theta not navigational, i.e. 0 is east and 
 values increase counter-clockwise from there. Reducing the angle
 from a full circle does not make it draw like a pacman but rather
 with a straight chord-line from the start/end points on the circle
 as defined by the two angles.

STRING
 [COLOR R G B]
 LINE | ARC
 [LINE | ARC]
END
 String composed of any number of lines and/or arcs.

RING
 LINE | ARC
 [LINE | ARC]
END
 Ring composed of any number of lines and/or arcs. A ring is automatically
 closed with a line from last point to first point.

POLYGON
 [COLOR R G B]
 [FCOLOR R G B]
 RING
 [RING]
END
 A polygon is composed of any number of rings. The first ring is outer
 (clockwise) and others are inner (counter-clockwise).


------------------------------------

Note: restriction for clockwise/counter clockwise rings should be removed 
later and replaced by reordering in symbol library. Order is required by 
ps.map because PS uses even-odd rule for polygon filling.

Internal structure:
A symbol read into memory is stored in structure SYMBOL containing an array 
of parts (SYMBPART). Each part may be of type S_STRING or S_POLYGON, and 
contain an array of chains (SYMBCHAIN) where both STRING or POLYGONS are 
stored. One chain contains an array of elements (SYMBEL).