code: void difference(IntSet dest, IntSet compared) {
          int i = 0;
          int j = 0;
          while (i < size && j < compared.size) {
              int a = elements[i];
              int b = compared.elements[j];
              if (a < b) {
                  // our set has a value that is not in "compared"
                  dest.addInt(a);
                  i ++;
              } else if (a > b) {
                  // "compared" has a value that is not in our set.
                  j ++;
              } else {
                  // the sets match at this index.
                  i ++;
                  j ++;
              }
          }
          // anything left in our set is part of the delta and belongs in "dest".
          while (i < size) {
              dest.addInt(elements[i]);
              i ++;
          }
      }
docs: Adds to the set "dest" values that in this set but are not in the set
  "compared".