import proveit
# Prepare this notebook for defining the theorems of a theory:
%theorems_notebook # Keep this at the top following 'import proveit'.
from proveit import x, C
from proveit.logic import (
Forall, Not, in_bool, Equals, InClass, NotInClass)
%begin theorems
not_in_class_is_bool = Forall(
(x, C), in_bool(NotInClass(x, C)),
conditions=[in_bool(InClass(x, C))] )
unfold_not_in_class = Forall(
(x, C), Not(InClass(x, C)), conditions=[NotInClass(x, C)])
fold_not_in_class = Forall(
(x, C), NotInClass(x, C), conditions=[Not(InClass(x, C))])
%end theorems