static VF1<ThoughtSpace1> ts1_applyGroupingPrefix() {
  ret voidfunc(ThoughtSpace1 ts) {
    S s = ts.getInput();
    if (empty(s)) ret;
    for (S prefix : sentencePrefixes_notEmpty_rev(s))
      if (cic(ts.getDefinitionTexts(prefix),
        "a grouping prefix"))
      ts.add(ProposeRewrite(s, prefix + " " + curlyBrace(trim(dropPrefix(prefix, s)))));
  };
}