ArcTan(x,y)?

[jte]

As a general comment on atan2 / ArcTan and floating-point: this is one of the places where signed zeroes can light a clear path. (For those less familiar with the numerical details of the prime: some of the number systems used in the Prime have signed zeroes, some do not.)

Signed zeroes and atan2 / ArcTan can also surprise some programmers (depending on their numerical outlook).