; fortran-atan computes atan of one or two arguments returning a range of values
; compatible with standard fortran (particularly, IBM and MULTICS).
; These are -%pi/2 to %pi/2 for single arg atan, and -%pi to %pi for 2 arg
; atan.  LISP unfortunately returns values in the range 0 to 2%pi, and only
; takes two arguments.

(declare (fixnum n) (flonum (fortran-atan)))

(defun fortran-atan n
   (prog (x y fa) (declare (flonum x y fa))
	 (setq x (arg 1) y (cond ((= n 2)(arg 2))(t 1.0))
	       fa (atan x y))
	 (return
	  (cond ((= n 1)
		 (cond ((> fa 3.14159265) (-$ fa (*$ 2.0 3.14159265)))
		       (t fa)))
		(t (cond ((> fa 3.14159265)
			  (-$ fa (*$ 2.0 3.14159265)))
			 (t fa)))))))