updates needed for total expansion
make transitivity push-back proof precise
make transitivity push-back proof precise (part 2: prepend_chain)
start fixing exp_preserves_sub_pr
use label instead of nat in the history of exp
Lemma nonempty_exp_doesnt_need_history
adapt case subtyp_sel_l of exp_preserves_sub to exp with history, which

requires 3 new lemmas:
* nonempty_exp_doesnt_need_history (already proven in last commit)
* supertype_of_top_expands_to_decs_nil (TODO)
* invert_empty_expansion (TODO)