I'm looking to simulate two joints in a finger actuated by a single motor,
where the outer link rotates at a different rate than the inner link.  It
looks like a custom transmission is the best method for doing this.  Is this
correct, and if so, are there any non-obvious tricks for coding up a custom
transmission?  I've looked through the 3 transmissions in
pr2_mechanism_model.

Thanks

Andrew Mor