Hey, I'm making my first proper 2D physics engine (I guess a kind of right of passage).
I've started by doing vector to vector constraints.
It works at the moment but it uses a kind of Master/Slave thing where one vector can move and move the other but when the slave vector moves it doesn't move the master (like a pendulum).
Here's the code:
//Finds out the angle between the vectors
angle = 90 + a.getAngle(b);