code: boolean isEmpty() {
      if (tc.isEntryEnabled())
        tc.entry(this,cclass, "isEmpty");
      boolean ans = super.isEmpty() && !haveEqualityMatches();
      if (tc.isEntryEnabled())
        tc.exit(this,cclass, "isEmpty", new Boolean(ans));
      return ans;        
    }
docs: Override isEmpty to check whether children is empty