@staticmethod
def _add_block(current_block, new_block):
    """
        Helper method that adds `new_block` to `current_block`, if the latter
        is not `None`. Otherwise, create a new block from `new_block`.

        Parameters
        ----------
        current_block: None or vermouth.molecule.Block
        new_block: vermouth.molecule.Block

        Returns
        -------
        vermouth.molecule.Block
            The combination of `current_block` and `new_block`
        """
    if not current_block:
        current_block = new_block.to_molecule(default_attributes={})
    else:
        current_block.merge_molecule(new_block)
    return current_block
