%N arctan %F $MIR/src/subs/utilities.for
%D Arctangent function
%P bpw
%: utilities
%B
      real   function arctan( y, x )
      real   y, x

 This is like the atan2 intrinsic function, except that it gives the
 proper answer if x=0 (pi/2 for y>0 and 3pi/2 for y<0)

 Input:
   x,y:     x and y coordinates
