code: @SuppressWarnings({"WeakerAccess", "unused"}) // For library users
      public Searcher addFacet(String... attributes) {
          for (String attribute : attributes) {
              final Integer value = facetRequestCount.get(attribute);
              facetRequestCount.put(attribute, value == null ? 1 : value + 1);
              if (value == null || value == 0) {
                  facets.add(attribute);
              }
          }
          rebuildQueryFacets();
          return this;
      }
docs: Adds one or several attributes to facet on for the next queries.
  
  @param attributes one or more attribute names.
  @return this {@link Searcher} for chaining.