@Override
    public DoubleStream flatMapToDouble(Function<? super Map.Entry<K, V>, ? extends DoubleStream> mapper) {
        return inner.flatMapToDouble(requireNonNull(mapper));
    }
Returns an {@code DoubleStream} consisting of the results of replacing
each element of this stream with the contents of a mapped stream produced
by applying the provided mapping function to each element.  Each mapped
stream is {@link java.util.stream.BaseStream#close() closed} after its
contents have placed been into this stream.  (If a mapped stream is
{@code null} an empty stream is used, instead.)
<p>
This is an intermediate operation.

@param mapper  a non-interfering, stateless function to apply to each 
               element which produces a stream of new elements
@return        the new stream
@throws NullPointerException if any of the provided parameters are {@code null}

@see #flatMap(Function)
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java