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.