code: @Override
      public boolean add(X x) {
          if (!allowElementsWithoutIdentification)
              throw new IllegalArgumentException("It is not allowed to put Elements without Identification in this Set");
  
          return map.put(x, placeholder) == null;
      }
docs: Adds an Element to the Set
  @param x the Element
  @return true if this set did not already contain the specified element
  @throws java.lang.IllegalArgumentException if it is not allowed to put Elements without Identification in this
  Set