var weldJoint = new SymbolicConstraint("
body b1
body b2

vector anchor1
vector anchor2
scalar phase

let r1 = relative b1.rot anchor1 in
let r2 = relative b2.rot anchor2 in
{ (b2.pos + r2) - (b1.pos + r1)
    b2.rot - b1.rot - phase     }
");

weldJoint.setBody("b1", ...);
weldJoint.setBody("b2", ...);
weldJoint.setVector("anchor1", new Vec2(...));
weldJoint.setVector("anchor2", new Vec2(...));
weldJoint.setScalar("phase", ...);

weldJoint.space = space;

trace(weldJoint.debug());