code: public Optional<Identification> getIdentificationFor(X x) {
          Identification identification = map.get(x);
          if (identification == null || identification.equals(placeholder)) {
              return Optional.empty();
          } else {
              return Optional.of(identification);
          }
      }
docs: returns the associated Identification (if it was added with an identification)
  @param x the Element
  @return the identification or an Empty Optional