The circles of Apollonius are formed by two pencils:
The first is the ratio r of the distances from one point P to two fixed points C and D, denoted as follows:
{ P{x, y} | dPC / dPD = r }
The second is the geometric locus of the P{x, y} that form an inscribed
theta angle with the fixed points C and D, it is denoted as follows:
{ P{x, y} | angle CPD = theta }.