public Set<String> getSummaryFields() {
        return summaryFields;
    }
Return the set of explicitly requested fields. Returns an empty set if no
fields are specified outside of summary classes. The returned set is
mutable and fields may be added or removed before passing on the query.

@return the set of names of requested fields, never null
container-search/src/main/java/com/yahoo/search/query/Presentation.java