code: private static void vecswap(Quicksortable q, int a, int b, int n) {
          for (int i = 0; i < n; i++, a++, b++)
              q.swap(a, b);
      }
docs: Swaps x[a .. (a+n-1)] with x[b .. (b+n-1)].
  
  @param q The quicksortable.
  @param a The first pointer to swap
  @param b The second pointer to swap
  @param n The number of elements to swap