code: public void add(IntDoubleVector other) {
          if (other instanceof IntDoubleUnsortedVector) {
              IntDoubleUnsortedVector vec = (IntDoubleUnsortedVector) other;
              for (int i=0; i<vec.top; i++) {
                  this.add(vec.idx[i], vec.vals[i]);
              }
          } else {
              //  TODO: Add special case for IntDoubleDenseVector.
              other.iterate(new SparseBinaryOpApplier(this, new Lambda.DoubleAdd()));
          }
      }
docs: Updates this vector to be the entrywise sum of this vector with the other.