Spherical Cuts from 4 points

        int pts[] = expandpointgroup(0, 'select');

vector p0 = point(0, 'P', pts[0]);
vector p1 = point(0, 'P', pts[1]);
vector p2 = point(0, 'P', pts[2]);
vector p3 = point(0, 'P', pts[3]);

float mask = predicate_insphere(p0, p1, p2, p3, v@P);

f@mask = mask;