Result is atan(Y, X)
    which should be preferred for portability.
It is similar to calculating the arc tangent of Y/X, except that the signs of both arguments are used to determine the quadrant of the result. The result lies in the interval -pi..pi. The operation is valid even if X is zero, in which case the result is pi/2 or -pi/2. One application is the conversion of cartesian to polar coordinates, where this function computes the angle component (in radians).
In coroutining mode, if X or Y is uninstantiated, the call to atan/3 is delayed until both variables are instantiated.
Success:
      atan( 0.0, -1.0, Result).	(gives Result =  3.141592)
      atan( 1.0, -1.0, Result).	(gives Result =  2.356194)
      atan( 1.0,  0.0, Result).	(gives Result =  1.570796)
      atan( 1.0,  1.0, Result).	(gives Result =  0.785398)
      atan( 0.0,  0.0, Result).	(gives Result =  0.0)
      atan(-1.0,  1.0, Result).	(gives Result = -0.785398)
      atan(-1.0,  0.0, Result).	(gives Result = -1.570796)
      atan(-1.0, -1.0, Result).	(gives Result = -2.356194)
      atan(-0.0, -1.0, Result).	(gives Result = -3.141592)
      atan( 7.0,  7.0, Result).	(gives Result =  0.785398)
Fail:
      atan(1.55741, 0.0, 1).
Error:
      atan(A, 0.0, 6.0).              (Error 4).
      atan(1 + 0.55741, 1.0, R).      (Error 24).