All elementary joints should inherit from this base model, i.e., joints that are directly defined by equations, provided they compute either the rotation object of frame_b from the rotation object of frame_a and from relative quantities (or vice versa), or there is a constraint equation between the rotation objects of the two frames. In other cases, a joint object should inherit from Interfaces.PartialTwoFrames (e.g., joint Spherical, because there is no constraint between the rotation objects of frame_a and frame_b or joint Cylindrical because it is not an elementary joint).
This partial model provides two frame connectors, a "Connections.branch" between frame_a and frame_b, access to the world object and an assert to check that both frame connectors are connected.