=over
=item atan2 Y,X
X X X X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the C
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for C is implementation-defined; consult
your atan2(3) manpage for more information.
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