def find_prefix_match(prefix, choices, start_index=0):
    """Return index of element in 'choices' that starts with 'prefix'.

    prefix       the prefix string to search for
    choices      list of choces strings
    start_index  the index to start searching 'choices' at

    Returns an index into 'choices' of unique find.
    Returns 'None' if no match found.
    """

    for (i, c) in enumerate(choices[start_index:]):
        if c.startswith(prefix):
            return i + start_index
    return None