(* This module contains a test of the ATAN 
   function in a constraint. When dragging, the 
   constraint is maintained, but when the point 
   "b" is adjusted, it is not. 

   NOTE: A fix to the NonLinearSolve.m3 module on 
   26-Sept-94 corrected this problem [heydon]. *)

CONST Theta = 1;

PRED FixedAngle(a, b) IS 
  (E ax, ay, bx, by :: 
    a = (ax, ay) AND 
    b = (bx, by) AND 
    Theta = ATAN(by - ay, bx - ax)) 
END;

FUNC res = Atan(a, b) IS 
  (E ax, ay, bx, by :: 
    a = (ax, ay) AND 
    b = (bx, by) AND 
    res = ATAN(by - ay, bx - ax)) 
END;

PROC Cmd0() IS 
  IF 
    VAR a = (27.27, 28.82), b ~ (74.98, 103.1) IN 
      FixedAngle(a, b) -> 
        PS.MoveTo(a); 
        PS.LineTo(b); 
        PS.Type(b, Unparse.Value(Atan(a, b))) 
    END 
  FI 
END;