"body b1
body b2
body c

vector anchor1
vector anchor2
vector canchor1
vector canchor2
scalar restlength

let r1 = relative b1.rot anchor1 in
let r2 = relative b2.rot anchor2 in
let c1 = relative c.rot canchor1 in
let c2 = relative c.rot canchor2 in

mag((b2.pos + r2) - (c.pos + c2)) + mag((b1.pos + r1) - (c.pos + c1)) - restlength"