public LongStream mapToLong(ToLongBiFunction<? super K, ? super V> mapper) {
        requireNonNull(mapper);
        return inner.mapToLong(e -> mapper.applyAsLong(e.getKey(), e.getValue()));
    }
Returns a {@code LongStream} consisting of the results of applying the
given function to the elements of this stream.
<p>
This is an intermediate operation.

@param mapper  a non-interfering, stateless function to apply to each 
               element
@return        the new stream
@throws NullPointerException if any of the provided parameters are {@code null}
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java