static L<GazelleTree> gazelle_reasonWithPreprocessing(S content, O... _) {
  S userName = stringPar userName(_);
  GazelleEvalContext ctx = cast optPar ctx(_);
  F0<GazelleEvalContext> contextMaker = optPar contextMaker(_, f<GazelleEvalContext> gazelle_stdChatEvalContext);
  bool debug = boolPar debug(_);
  
  // preprocess
  
  L<GazelleTree> l1 = ll(GazelleTree(contextMaker!, content));
  addAll(l1, dm_gazelle_reasonAboutChatInput_v2(userName, content, 
    paramsPlus(_, acceptablePurposes := litset('preprocess))));
  
  // TODO: uniquify lines?
  
  if (debug) print("Got " + n_entries(l1) + " after preprocessing.");
  
  new L<GazelleTree> l2;
  
  for (GazelleTree tree : l1)
    addAll(l2, dm_gazelle_reasonAboutChatInput_v2(userName, tree.line, paramsPlus(tree := tree, _)));
  
  ret l2;
}