atan

[procedure] atan z

[procedure] atan y x

R5RS, R6RS: arctan. two argument version calculates (angle (make-rectangular x y)).

R6RS specifies these boundary cases:

 (atan -inf.0)  => -π/2
 (atan +inf.0)  =>  π/2

SRFI-94: two arguments version signals error if y or x is not real.