*
   * @param values the values to use when constructing the {@code Map}
   * @param keyFunction the function used to produce the key for each value
   * @return a map mapping the result of evaluating the function {@code keyFunction} on each value
   *     in the input collection to that value
   * @throws IllegalArgumentException if {@code keyFunction} produces the same key for more than one
   *     value in the input collection