'Execute the optree. When calling `execute()`, there must be one and only
one `assign` operation at the very top of the op-tree. The corresponding
backend\'s execute function will be called.'

def execute(self):

    assert (self[0]['op'] == 'assign')
    backend = self[1].backend
    if isinstance(backend, Backend):
        return backend.execute(self)
    else:
        raise NotImplementedError()
