Proof of proveit.logic.sets.subtraction.all_but_one_membership_unfolding theorem

import proveit
theory = proveit.Theory() # the theorem's theory

%proving all_but_one_membership_unfolding

With these allowed/disallowed theorem/theory presumptions (e.g., to avoid circular dependencies), we begin our proof of
all_but_one_membership_unfolding:
(see dependencies)