// e.g. in = "very $x", out = "$x"
svoid philosophyBot1_addSimplificationRule(PhilosophyBot1 bot, S in, S out) {
  Set<PairS> replacements = cast bot.getExtension("simplification rules");
  if (replacements == null) {
    bot.addExtension("simplification rules", replacements = new Set);
    Set<PairS> _replacements = replacements;
    bot.addDeepTransformer(s -> {
      for (PairS p : _replacements)
        try answer simpleRewriteWithDollarVars(p.a, p.b, s);
      null;
    });
  }
  replacements.add(pair(in, out));
}