How to write an ATan2( ) function?

Got it. I didn't realize it was such a simple thing.

Procedure.f ATan2(y.f,x.f)
a.f=ATan(y.f/x.f)
If x.f<0.0
a.f=180.0+a.f
EndIf
If a.f>180.0
a.f=a.f-360.0
EndIf
ProcedureReturn a.f
Endprocedure