code: public BigInteger getAndAdd(BigInteger delta) {
          while (true) {
          	BigInteger current = get();
          	BigInteger next = current.add(delta);
              if (compareAndSet(current, next))
                  return current;
          }
      }
docs: Atomically adds the given value to the current value.
  
  @param delta the value to add
  @return the previous value