def group_with_decoys(peptides, proteins):
    """Retrieve the protein group in the case where the FASTA has decoys.

    Parameters
    ----------
    peptides : pandas.DataFrame
        The peptide dataframe.
    proteins : a Proteins object

    Returns
    -------
    pandas.Series
        The protein group for each peptide.
    """
    return peptides["stripped sequence"].map(proteins.peptide_map.get)