Lists: Solved exercise bag_theorem.
Lists: Proved cats0, part of exercise list_exercises.
Lists: Proved rev_rcons, part of exercise list_exercises.
Lists: Proved revK, part of exercise list_exercises.
Lists: Proved catA4, part of exercise list_exercises.
Lists: Proved rcons_as_cat, part of exercise list_exercises.
Lists: Proved rev_cat, part of exercise list_exercises.
Lists: Proved nonzeros_cat, part of exercise list_exercises.
Lists: Tweaked the proof of nonzeros_cat.