Is bool boolval (#456)

* add BoolVal to check

* delete unnecessary linebreaks

* remove unused import

cpmpy/expressions/utils.py

@@ -24,15 +24,16 @@
 
 import numpy as np
 import math
-from collections.abc import Iterable # for _flatten
-from itertools import chain, combinations
+from collections.abc import Iterable  # for flatten
+from itertools import combinations
 from cpmpy.exceptions import IncompleteFunctionError
 
 
 def is_bool(arg):
     """ is it a boolean (incl numpy variants)
     """
-    return isinstance(arg, (bool, np.bool_))
+    from cpmpy import BoolVal
+    return isinstance(arg, (bool, np.bool_, BoolVal))
 
 
 def is_int(arg):