import proveit
from proveit import l, x, A
from proveit.numbers import two
from proveit.logic import bools_def, TRUE, FALSE
from proveit.logic.booleans import in_bool_def
from proveit.logic.sets.enumeration import enum_set_def
theory = proveit.Theory() # the theorem's theory
%proving in_bool_is_bool
in_bool_def
in_bool_def_spec = in_bool_def.instantiate({A:A})
in_bool_def_r_h_s_in_bool = in_bool_def_spec.rhs.deduce_in_bool()
in_bool_def_spec.sub_left_side_into(in_bool_def_r_h_s_in_bool)
%qed