Symbol is stored as text file (one file for each symbol)
in GISDBASE/etc/symbol/group/name or mapset/symbol/group/name
S_read() searches first in mapsets and then in GISDBASE
(symbol in GISDBASE may be overwritten by symbol 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

Symbol file is composed of objects, each object starts by keyword and ends
by new line or 'END'. Some objects may contain another objects.

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

STRINGS and POLYGONS are drawn by default (specified as module option like d.vect color=)
color and fill color. This color may be over written by COLOR and FCOLOR.

---- DESCRIPTION OF OBJECTS ----
In following description is used 
  N - integer number
  R G B - RGB color as 3 numbers 0-255
  X,Y - coordinates (double)

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, polygone outline is not drawn.

FCOLOR NONE | R G B
 Fill color used for polygon. If NONE is used, polygone 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 (A1/A2) and
 oprionaly direction (C for clockwise, default counter clockwise)

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. Ring is automaticaly
 closed by line from last point to first point

POLYGON
 [COLOR R G B]
 [FCOLOR R G B]
 RING
 [RING]
END
 Polygon composed of any number of rings. First ring is outer (clockwise) and
 others are inner (counterclockwise).

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

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 array of parts (SYMBPART).
Part may be of type S_STRING or S_POLYGON, and contains array of chains (SYMBCHAIN) where 
both STRING or POLYGONS are stored. One chain contains array of elements (SYMBEL).