public static <T> ToDouble<T> doublePlusDouble(ToDouble<T> first, ToDouble<T> second) {
        class DoublePlusDouble extends AbstractPlus<T, ToDouble<T>, ToDouble<T>> implements ToDouble<T> {
            private DoublePlusDouble(ToDouble<T> first, ToDouble<T> second) {
                super(first, second);
            }

            @Override
            public double applyAsDouble(T object) {
                return firstInner.applyAsDouble(object)
                    + secondInner.applyAsDouble(object);
            }
        }

        return new DoublePlusDouble(first, second);
    }
Creates and returns an expression that takes the result of the two
expressions and add them together.

@param first   the first input expression
@param second  the second input expression
@param <T>     the input type
@return        the new expression
runtime-parent/runtime-compute/src/main/java/com/speedment/runtime/compute/internal/expression/PlusUtil.java