code: void addLower(MutableBigInteger addend, int n) {
          MutableBigInteger a = new MutableBigInteger(addend);
          if (a.offset + a.intLen >= n) {
              a.offset = a.offset + a.intLen - n;
              a.intLen = n;
          }
          a.normalize();
          add(a);
      }
docs: Adds the low {@code n} ints of {@code addend}.