docs: private static Selector simplifyEQ(Selector sel0, Selector sel1) {
      if (sel0.getType() != Selector.BOOLEAN)
        return new OperatorImpl(Selector.EQ, sel0, sel1);
      else
        return makeOR(makeAND(simplifyTree(sel0), simplifyTree(sel1)),
                      makeAND(simplifyNOT(sel0), simplifyNOT(sel1)));
    }