var pivot = new SymbolicConstraint("
body b1, b2
vector anchor1, anchor2
(relative b2.rot anchor2 + b2.pos) - (relative b1.rot anchor1 + b1.pos)
");

pivot.setBody("b1", ...);
pivot.setBody("b2", ...);
pivot.setVector("anchor1", ...);
pivot.setVector("anchor2", ...);
pivot.space = ...;