In this week's precept, we took the RANGE module type and brainstormed
some possible representations. Using the (lo,hi) implementation, we
then implemented the API, wrote a function for the
representation invariant and determined our proof obligations for the
invariant on that implementation. Finally, we wrote some of the
proofs.

As your suggested exercise this week, for additional practice you
should come up with your own representation of a range (whether one
that we brainstormed or something of your own creation), implement it,
write a function for the representation invariant, and do the same
proofs to satisfy the proof obligation.
