code: public UnicodeSet addAll(UnicodeSet c) {
          checkFrozen();
          add(c.list, c.len, 0);
          strings.addAll(c.strings);
          return this;
      }
docs: Adds all of the elements in the specified set to this set if
  they're not already present.  This operation effectively
  modifies this set so that its value is the <i>union</i> of the two
  sets.  The behavior of this operation is unspecified if the specified
  collection is modified while the operation is in progress.
  
  @param c set whose elements are to be added to this set.