def search_one(self, keyword, arg=None, children=None):
        """Return receiver's substmt with `keyword` and optionally `arg`.
        """
        if children is None:
            children = self.substmts
        for ch in children:
            if ch.keyword == keyword and (arg is None or ch.arg == arg):
                return ch
        return None
Return receiver's substmt with `keyword` and optionally `arg`.