sym4mm.icn: Program to draw symmetrically

This program draws with the eight symmetries of the square - 4mm
symmetry.

It is based on a simple drawing program by Gregg Townsend.

Pressing the left mouse button draws a point.  Dragging with the left mouse
button depressed draws a line.  Pressing and dragging with the middle mouse
depressed shows a dashed straight line, which is drawn solid when
the middle mouse button is released.  Dragging with the right mouse
button depressed erases in the vicinity of the mouse pointer.

Typing "f" toggles restriction of drawing to the "generating region"
which is shaded when drawing is restricted.

Typing "g" toggles the grid lines.

Typing "p" toggles the background in the generating region.

Typing "c" clears the window.

Typing "s" takes a snapshot, writing a GIF file.  File names begin with
a prefix, followed by three digits starting at 000 and increasing, and
terminated by .GIF.

Typing "q" terminates the session.

Grid lines and shading are only provided for servers that support mutable
colors.

The options supported are:

     -w i    width of the window, default 512
     -h i    height of the window, default 512
     -s i    size of square, default 512x512; supersedes -w and -h
     -p s    prefix for image files, default "sym"

Note:  Although the window does not have to be square, the application is
designed to work with a square window.