Pic's `with' attribute now accepts positions.
* src/preproc/pic/pic.y: Make `.', BOX, CIRCLE, ELLIPSE, ARC, LINE,
ARROW, SPLINE, and `[' left-associative tokens to fix shift/reduce
conflicts.
(object_spec): Add rule for `WITH' and `position'.
(relative_path): Give `corner' the precedence of `CHOP'.
* src/preproc/pic/object.h (path): New members `pos' and
`is_position'.
* src/preproc/pic/object.cc: Updated initializers of `path'.
(path::follow): Handle `is_position'.
* doc/pic.ms: Completely updated grammar description.
Many typographical improvements.