'Return the value of the expression as constant or raise
:exc:`Impossible` if this was not possible.
An :class:`EvalContext` can be provided, if none is given
a default context is created which requires the nodes to have
an attached environment.
.. versionchanged:: 2.4
the `eval_ctx` parameter was added.'

def as_const(self, eval_ctx=None):

    raise Impossible()
