@Override
    public DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper) {
        requireNonNull(mapper);
        if (STRICT) {
            return toStream().flatMapToDouble(mapper);
        }
        return mapper.apply(element);
    }
Creates and returns an optimized sequential {@link Stream} implementation
with only a single element if the element is non-null. if the element is
null, an empty Stream is returned.

@param element the single element or null
@param <T> the type of the stream elements
@return a singleton sequential stream
common-parent/singletonstream/src/main/java/com/speedment/common/singletonstream/SingletonStream.java