def add_fixed_mods(seqs:list, mods_fixed:list, **kwargs)->list:
    """
    Adds fixed modifications to sequences.
    Args:
        seqs (list of str): sequences to add fixed modifications
        mods_fixed (list of str): the string list of fixed modifications. Each modification string must be in lower case, except for that the last letter must be the modified amino acid (e.g. oxidation on M should be oxM).
    Returns:
        list (of str): the list of the modified sequences. 'ABCDEF' with fixed mod 'cC' will be 'ABcCDEF'.
    """
    if not mods_fixed:
        return seqs
    else:
        for mod_aa in mods_fixed:
            seqs = [seq.replace(mod_aa[-1], mod_aa) for seq in seqs]
        return seqs