If th is a theorem of the form returned by the function
define_new_type_bijections:
|- (!a. abs(rep a) = a) /\ (!r. P r = (rep(abs r) = r))
then prove_rep_fn_one_one th proves from this theorem that the
function rep is one-to-one, returning the theorem:
|- !a a'. (rep a = rep a') = (a = a')