modlines.icn: Procedure to produce trace of modular lines

link modlines
August 3, 2000; Ralph E. Griswold
This file is in the public domain.

For a description of the method used here, see

     Geometric and Artistic Graphics; Design Generation with
     Microcomputers, Jean-Paul Delahaye, Macmillan, 1987, pp. 90-95.