Guyslain: improving section 2, invariants
Merge branch 'master' of https://github.com/aspiwack/fulltrees
guyslain: removing sentence on height of trees increasing by one at each pass. This is not precise enough: all trees do not have the same height.
guyslain: renamig variables one and two into odd and even, for clarity
guyslain: size -> length (* precision *)
full.ml: no more non-empty lists.
Full.v: no-more non-empty lists.
full.ml: balance_powerlist ↝ loop.
Update Proof.v for the non-empty-list free implementation.