MODULE Line;

(* Procedures and predicates for 
   lines and line segments. *)

IMPORT Geometry, PS;

(* A line segment "[a, b]" is 
   defined by its two endpoints "a" 
   and "b". *)

PROC Draw(a, b) IS 
  PS.MoveTo(a); PS.LineTo(b) 
END;

UI PointTool(Draw);

(* Add the line segment "[a, b]" to 
   the current path. The current 
   point becomes "b". *)

PRED On(p, a, b) IS 
  Geometry.Colinear(a, p, b) 
END;

UI PointTool(On);

(* The point "p" is on the line 
   "[a, b]". *)