def get_mod_mappings(mappings):
    """
    Returns a dict of all known modification mappings.

    Parameters
    ----------
    mappings: collections.abc.Iterable[vermouth.map_parser.Mapping]
        All known mappings.

    Returns
    -------
    dict[tuple[str], vermouth.map_parser.Mapping]
        All mappings that describe a modification mapping.
    """
    out = {}
    for mapping in mappings:
        if mapping.type == 'modification':
            out[mapping.names] = mapping
    return out