code: public UnicodeSet removeAll(UnicodeSet c) {
          checkFrozen();
          retain(c.list, c.len, 2);
          strings.removeAll(c.strings);
          return this;
      }
docs: Removes from this set all of its elements that are contained in the
  specified set.  This operation effectively modifies this
  set so that its value is the <i>asymmetric set difference</i> of
  the two sets.
  
  @param c set that defines which elements will be removed from
  this set.