import proveit
from proveit.logic.sets.inclusion import proper_subset_def
theory = proveit.Theory() # the theorem's theory
%proving relax_proper_subset
# pull in the axiomatic definition of proper subset
proper_subset_def
proper_subset_def_inst = proper_subset_def.instantiate()
proper_subset_def_inst.derive_right_via_equality(assumptions=[proper_subset_def_inst.lhs])
%qed