MODULE DragResponse;

(* This is a simple module to show how 
   dragging sometimes tracks the mouse 
   well and sometimes does not, even when 
   the two cases seem symmetric. *)

PROC Quad(a, b, c, d) IS 
  IF 
    PS.MoveTo(a); 
    PS.LineTo(b); 
    PS.LineTo(c); 
    PS.LineTo(d); 
    PS.Close() 
  FI 
END;

(* Without any constraints on "g", the 
   point "e" is easy to drag, but the 
   symmetric point "f" is not. If we add 
   the constraint "f HOR g", we can use 
   "g" as a handle to drag "f". 

   However, once this constraint is in 
   force, it is no longer easy to drag 
   "e", even if "g" is unfrozen. 
   Surprisingly, *adjusting* "e" in this 
   case works quite well. How come? *)

PROC Cmd0() IS 
  IF 
    VAR 
      a = (-170.41092, 132.14568), b = (-88.89553, 132.14568), 
      c ~ (91.09341, 132.14568), d = (172.6088, 132.14568), 
      e ~ (-39.658745, -144.57802), f ~ (41.856644, -144.57802), 
      g ~ (83.312004, -142.93307) 
    IN 
      a HOR b AND 
      a HOR c AND 
      a HOR d AND 
      e HOR f AND 
      (a, e) PARA (b, f) AND 
      (e, c) PARA (f, d) AND 
      (a, e) CONG (f, d) -> Quad(a, e, f, b); Quad(e, f, d, c) 
    END 
  FI 
END;