a version of arctangent taking two real floating-point arguments. For
real floating
x and
y,
atan2 y x
computes the angle (from the positive x-axis) of the vector from the
origin to the point
(x,y).
atan2 y x returns
a value in the range [
-pi,
pi]. It follows the
Common Lisp semantics for the origin when signed zeroes are supported.
atan2 y 1, with
y in a type that is
RealFloat, should return the same value as
atan
y. A default definition of
atan2 is provided, but
implementors can provide a more accurate implementation.