code: @Override
    public ValNum apply(Env env, Env.StackHelp stk, AstRoot asts[]) {
      Frame fr = stk.track(asts[1].exec(env)).getFrame();
      if (fr.numCols() != 1 || !fr.anyVec().isCategorical())
        throw new IllegalArgumentException("mode only works on a single categorical column");
      return new ValNum(mode(fr.anyVec()));
    }
docs: (mode ary)