code: public DoubleAdder add(final DoubleAdder value) {
  		addWithCompensation(value._sum);
  		addWithCompensation(value._compensation);
  		_simpleSum += value._simpleSum;
  		return this;
  	}
docs: Add the given {@code value} to this adder, using the
  <a href="http://en.wikipedia.org/wiki/Kahan_summation_algorithm">Kahan
  summation algorithm</a>
  
  @param value the {@code value} to add
  @return {@code this} adder, for command chaining
  @throws java.lang.NullPointerException if the given {@code value} is
  {@code null}