lispdoc  results for phase 
(phase number)  Function: Return the angle part of the polar representation of a complex number. For complex numbers, this is (atan (imagpart number) (realpart number)). For noncomplex positive numbers, this is 0. For noncomplex negative numbers this is PI.

Mentioned in: CLtL2  12.5.1. Exponential and Logarithmic Functions CLtL2  12.5.2. Trigonometric and Related Functions CLtL2  12.5.3. Branch Cuts, Principal Values, and Boundary Conditions in the Complex Plane HyperSpec  Function PHASE  
By Bill Moorier 