code: @Override public ValFrame apply(Env env, Env.StackHelp stk, AstRoot asts[]) {
      Frame fr = stk.track(asts[1].exec(env)).getFrame();
      int[] cols = ((AstParameter)asts[2]).columns(fr.names());
      int[] sortAsc;
      if (asts[3] instanceof AstNumList)
        sortAsc = ((AstNumList) asts[3]).expand4();
      else
        sortAsc = new int[]{(int) ((AstNum) asts[3]).getNum()};  // R client can send 1 element for some reason
      
      assert sortAsc.length==cols.length;
      return new ValFrame(Merge.sort(fr,cols, sortAsc));
    }
docs: (sort ary [cols] [int])