def insert_tags(block, i, offset, new_tokens):
    """
        Insert new tokens at a given offset in a given string inside a list of strings.
        Return change in length of block.
        Example:
        >>> block = ['foo', 'bar']
        >>> assert insert_tags(block, 1, 2, [['new'], ['stuff']]) == 3
        >>> assert block == ['foo', 'ba', ['new'], ['stuff'], 'r']
    """
    text = block[i]
    to_insert = [token for token in [text[:offset]]+new_tokens+[text[offset:]] if token]
    block[i:i + 1] = to_insert
    return len(to_insert) - 1