def is_base_dir(d):
    '''True if the dir is valid and it contains a dir called s2'''
    if not dir_param_valid(d): # pragma: no cover
        raise 
    else:
        mfn = os.path.join(d,'s2') #marker name. it must be a directory.
        return os.path.isdir(mfn)
True if the dir is valid and it contains a dir called s2