'Return the value of the expression as constant or raise
:exc:`Impossible` if this was not possible:
>>> Add(Const(23), Const(42)).as_const()
65
>>> Add(Const(23), Name(\'var\', \'load\')).as_const()
Traceback (most recent call last):
Impossible
This requires the `environment` attribute of all nodes to be
set to the environment that created the nodes.'

def as_const(self):

    raise Impossible()
