is_seq(arg) yields True if arg is a sequential collection otherwise False; i.e., it must be a
      list, tuple, persistent vector, persistent list, or numpy array.
    
    Note that strings are not considered sequences.

def is_seq(arg):
    '''
    is_seq(arg) yields True if arg is a sequential collection otherwise False; i.e., it must be a
      list, tuple, persistent vector, persistent list, or numpy array.
    
    Note that strings are not considered sequences.
    '''
    return isinstance(arg, (list_type, tuple_type, pyr.PVector, pyr.PList)) or is_nparray(arg)