code: public PartialConverter addPartialConverter(PartialConverter converter)
              throws SecurityException {
          
          checkAlterPartialConverters();
          if (converter == null) {
              return null;
          }
          PartialConverter[] removed = new PartialConverter[1];
          iPartialConverters = iPartialConverters.add(converter, removed);
          return removed[0];
      }
docs: Adds a converter to the set of converters. If a matching converter is
  already in the set, the given converter replaces it. If the converter is
  exactly the same as one already in the set, no changes are made.
  <p>
  The order in which converters are added is not relevant. The best
  converter is selected by examining the object hierarchy.
  
  @param converter  the converter to add, null ignored
  @return replaced converter, or null