message: reduce peak memory consumption during preprocessing (instantiate_undef)
diff: @@ -237,11 +237,13 @@ static void instantiate_undef(const Input *in, map<expr, expr> &instances,
  expr nums[2] = { expr::mkUInt(0, 1), expr::mkUInt(1, 1) };
  - for (auto &[e, v] : instances) {
  + for (auto I = instances.begin(); I != instances.end();
  + I = instances.erase(I)) {
  + auto &[e, v] = *I;
  for (unsigned i = 0; i < 2; ++i) {
  expr newexpr = e.subst(var, nums[i]);
  if (newexpr.eq(e)) {
  - instances2[move(newexpr)] = v;
  + instances2[move(newexpr)] = move(v);
  break;
  }
  