message: fml preprocessing: keep the first fml (non-undef/poison) rather than the last one
diff: @@ -215,7 +215,7 @@ static expr preprocess(Transform &t, const set<expr> &qvars,
  continue;
  // keep 'var' variables for counterexample printing
  - instances2[move(newexpr)] = v && var == nums[i];
  + instances2.try_emplace(move(newexpr), v && var == nums[i]);
  }
  }
  instances = move(instances2);
  