def pad_seq_with_mask(seq, mask):
    """Given a shorter sequence, expands it to match the padding in mask.

    Args:
        seq: String with length smaller than mask.
        mask: String of '+'s and '-'s used to expand seq.

    Returns:
        New string of seq but with added '-'s where indicated by mask.
    """
    seq_iter = iter(seq)
    new_seq = ""
    for m in mask:
        if m == "+":
            new_seq += next(seq_iter)
        elif m == "-":
            new_seq += "-"
    return new_seq