Returns a new, reduced "protToPeps" dictionary that does not contain
    entries present in "proteins".

    :param protToPeps: dict, for each protein (=key) contains a set of
        associated peptides (=value). For Example {protein: {peptide, ...}, ...}
    :param proteins: a list of proteinSet
    :returns: dict, protToPeps not containing entries from "proteins"

def _reducedProtToPeps(protToPeps, proteins):
    """Returns a new, reduced "protToPeps" dictionary that does not contain
    entries present in "proteins".

    :param protToPeps: dict, for each protein (=key) contains a set of
        associated peptides (=value). For Example {protein: {peptide, ...}, ...}
    :param proteins: a list of proteinSet
    :returns: dict, protToPeps not containing entries from "proteins"
    """
    return {k: v for k, v in viewitems(protToPeps) if k not in proteins}