@Override
    public Optional<Item> extractSingleChild() {
        return Optional.empty();
    }
A composite item which specifies semantics which are not maintained
if an instance with a single child is replaced by the single child.
Most composites, like AND and OR, are reducible as e.g (AND a) is semantically equal to (a).
This type functions as a marker type for query rewriters.

@author bratseth
container-search/src/main/java/com/yahoo/prelude/query/NonReducibleCompositeItem.java