def rearrange(letter, string, pos):
    """
    Function that generates new strings by inserting
    the letter in all possible positions within the string.

    Returns a list with new strings.
    """
    if pos == len(string):
        return [string + letter]
    else:
        current_word = string[: pos] + letter + string[pos: ]
        return [current_word] + rearrange(letter, string, pos + 1)