code: public Set<Integer> intKeySet() {
          return data.keySet().stream()
                  .map(Any2::getLeft)
                  .flatMap(opt -> opt.map(Stream::of).orElseGet(Stream::empty))
                  .collect(Collectors.toSet());
      }
docs: The numeric set of keys held by the resolver map.
  
  @return the numberic set of keys held by the resolver map.