def get_filter_tag(accession, decoy_string, count):
    """Checks if accession number corresponds to a decoy or contaminant.
    """
    tag = ''
    if 'CONT|' in accession or 'CONT_' in accession:
        tag = 'contaminant'
    if decoy_string in accession:
        tag = 'reversed'
    if count > 0:
        tag = 'redundant'
    return tag