message: eliminate expr copy
diff: @@ -283,7 +283,7 @@ static void check_refinement(Errors &errs, Transform &t,
  return;
  }
  - auto mk_fml = [&](const expr &refines) -> expr {
  + auto mk_fml = [&](expr &&refines) -> expr {
  // from the check above we already know that
  // \exists v,v' . pre_tgt(v') && pre_src(v) is SAT (or timeout)
  // so \forall v . pre_tgt && (!pre_src(v) || refines) simplifies to:
  @@ -291,7 +291,7 @@ static void check_refinement(Errors &errs, Transform &t,
  // \forall v . (pre_tgt && !pre_src(v)) -> [\exists v . pre_src(v)]
  // false
  if (refines.isFalse())
  - return refines;
  + return move(refines);
  return axioms &&
  preprocess(t, qvars, uvars, pre_tgt && pre_src.implies(refines));
  