public DoubleStream flatMapToDouble(BiFunction<? super K, ? super V, ? extends DoubleStream> mapper) {
        requireNonNull(mapper);
        return inner.flatMapToDouble(e -> mapper.apply(e.getKey(), e.getValue()));
    }
Returns an {@code DoubleStream} consisting of the results of replacing
each key-value pair 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 
               key-value pair 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