If we get a single positional argument, and only a single
        variable needs to be filled, fill that variable with
        the value from that positional argument.

def fill_arg(self, *args):
        """
        If we get a single positional argument, and only a single
        variable needs to be filled, fill that variable with
        the value from that positional argument.
        """
        missing = self.missing_vars()
        if len(args) == len(missing) == 1:
            self.setval(missing[0], args[0])