def cleanup_sequence(seq, valid):
    """Given a sequence (list or string) and a string describing the
       valid sequence characters, builds a new sequence containing only 
       valid sequence characters from the original sequence."""
    result = []
    for val in seq:
        if val in valid:
            result.append(val)
    if is_string(seq):
        return string.join(result, '')
    else:
        return result