Proved a variation of element_changed_by_inserting_smaller_key and adjusted split_insert_left to use the slighly changed, but  proved, lemma.
Merge branch 'master' of github.com:nicolaidahl/BPlusTrees
Merge branch 'master' of github.com:nicolaidahl/BPlusTrees
Proved the lemma insert_leaf_split_never_empty.