a2: CircumcircularArc[P2, M2, Q2]
  1. Ideal arcs
Consider the situation for which A and B are not collinear with O and both points lie on γ. In this case, the previous construction fails since both points are invariant under inversion about γ and inversion cannot be used to obtain the third point needed to construct the circle. The solution is to use a result from Euclidean geometry as illustrated in Figure 7.