Point3D.angle() is is NaN for the following points:
Point3D [x = -0.8944271909999159, y = 0.0, z = 0.4472135954999579] and
Point3D [x = -0.894427190999924, y = 4.061090000458082E-14, z = 0.4472135954999417]

Run attached code to reproduce this.

Internally acos is invoked on value that is slightly greater than 1 due to calculation errors: "c = 1.0000000000000002".