code: public int firstIndexOfDifference(Permutation other) {
          int r = 0;
          while ((r < values.length) && values[r] == other.get(r)) {
              r++;
          }
          return r;
      }
docs: Find an r such that this[r] != other[r].
  @param other the other permutation to compare with
  @return the first point at which the two permutations differ