code: private FDBigInteger mult(int i) {
          if (this.nWords == 0) {
              return this;
          }
          int[] r = new int[nWords + 1];
          mult(data, nWords, i, r);
          return new FDBigInteger(r, offset);
      }
docs: /*@
  @ requires this.value() == 0;
  @ assignable \nothing;
  @ ensures \result == this;
  @
  @  also
  @
  @ requires this.value() != 0;
  @ assignable \nothing;
  @ ensures \result.value() == \old(this.value() * UNSIGNED(i));
  @