'Call _from_base_type() if necessary.
If the value is a _BaseValue instance, unwrap it and call all
_from_base_type() methods.  Otherwise, return the value
unchanged.'

def _opt_call_from_base_type(self, value):

    if isinstance(value, _BaseValue):
        value = self._call_from_base_type(value.b_val)
    return value
