message: Check hit_half_memory_limit while substituting qvars
diff: @@ -200,6 +200,9 @@ static expr preprocess(Transform &t, const set<expr> &qvars0,
  for (auto &var : qvars0) {
  if (!var.isBool())
  continue;
  + if (hit_half_memory_limit())
  + break;
  +
  e = e.subst(var, true).simplify() &&
  e.subst(var, false).simplify();
  qvars.erase(var);
  