Transform the input state_name into a valid state in XMLBIF.
        XMLBIF states must start with a letter an only contain letters,
        numbers and underscores.

def _make_valid_state_name(self, state_name):
        """Transform the input state_name into a valid state in XMLBIF.
        XMLBIF states must start with a letter an only contain letters,
        numbers and underscores.
        """
        s = str(state_name)
        s_fixed = pp.CharsNotIn(pp.alphanums + "_").setParseAction(pp.replaceWith("_")).transformString(s)
        if not s_fixed[0].isalpha():
            s_fixed = "state" + s_fixed
        return s_fixed