static KeyExtractor.Factory forSingleMember(Function<Class<?>, Member> keyLookupFn) {
    Objects.requireNonNull(keyLookupFn, "keyLookupFn");
    return entityType -> new KeyExtractors.SingleMember(entityType, keyLookupFn.apply(entityType));
  }
Factory to extract key information using just one member (field or method) of a class.
In this case it will be a single key extractor using reflection.

@param keyLookupFn function extracting single member representing a key from a entity
criteria/common/src/org/immutables/criteria/backend/KeyExtractor.java