import proveit
from proveit import l, x, A
from proveit.logic import bools_def, TRUE, FALSE, InSet
theory = proveit.Theory() # the theorem's theory
%proving in_bool_def
bools_def
Ain_boolSet = InSet(A, bools_def.rhs)
in_bool_set_def = Ain_boolSet.definition()
bools_def.sub_left_side_into(in_bool_set_def)
%qed