Emulate the MuJoCo-WASM four-bar in numpy: an analytic closed-loop solver places crank, coupler and rocker for each input angle, traces the rocker tip, and compares it to the target arc while checking self-collision. You add the missing pivot by setting the coupler->rocker connection and tuning the input range. The final cell is the swept-path autograder: loop closure, net DOF, path RMSE, collisions, and range covered.