Get variables that are defined at the specified block.

        :param int block_addr:  Address of the block.
        :return:                A set of variables.

def get_variable_definitions(self, block_addr):
        """
        Get variables that are defined at the specified block.

        :param int block_addr:  Address of the block.
        :return:                A set of variables.
        """

        if block_addr in self._outstates:
            return self._outstates[block_addr].variables
        return set()