def describe_model(depends_on=None):
    """ get names of any conditional parameters included in the model
    """
    pkeys = list(depends_on)
    deplist = []
    if 'a' in pkeys:
        deplist.append('boundary')
    if 'tr' in pkeys:
        deplist.append('onset')
    if 'v' in pkeys:
        deplist.append('drift')
    if 'xb' in pkeys:
        deplist.append('xbias')
    if len(pkeys) > 1:
        depkeys = ' and '.join(deplist)
    else:
        depkeys = deplist[0]
    return depkeys
