What's the tag formal-systems for?
Just meta theory? Meta theory and formal proofs? Other options?
Edit: In view of the comments to this question and due to the fact there have been a lot of questions asking for formal proofs, should the tag (formal-proofs) be created? Most of these questions are either about natural deduction or Hilbert type system, There is a tag called (natural-deduction), but there's nothing for the rest. I think getting rid of (natural-deduction) and creating (formal-proofs) is appropriate. The other option is to include the tag (Hilbert-system) or similar.