Lists: Solved exercise list_design.
Lists: Proved count_member_nonzero, part of exercise bag_proofs.
Lists: Proved leqnSn, part of exercise bag_proofs.
Lists: Proved remove_decreases_count, part of exercise bag_proofs.
Lists: Solved exercise bag_count_sum.
Lists: Solved exercise rev_injective.
Lists: Solved exercise ohead.
Lists: Solved exercise option_elim_head.
Lists: Solved exercise eqn_natlist.
Lists: Solved exercise dictionary_invariant1.
Lists: Solved exercise dictionary_invariant2.