rho of tail = pred (rho of whole)
TODO List
Removed the plague of tail/neck like functions ..

.. accepting an instance argument of proof that len > 0. Using instance
arguments in functions might make work easier but can kill you when you
want to call cong
Misc.agda : Add EvenOdd and other lemmas
Lemma 2.4 : Prove \rho distance for first two classes of points e2