Two bugs were present before the patch:

- Formatting_gen would always be printed as "@{",
  so for example "@[foo@]" would be reprinted as "@{foo@]"

- The Formatting_gen payload would be printed as a string literal,
  escaping '%', while it is a raw string representation of a format;
  so for example "@[<%d>" would be reprinted as "@[<%%d>"

  (This second bug was spotted by Florian Angeletti)

- Currently the check is a no-op, because the "emit a warning if the
  check fails" was never implemented. (It would actually require some
  work to pass a source location there to emit a warning, so it's not
  trivial.)

- The check is implemented by calling `open_box_of_string` and
  catching the `Failure _` exception if that function fails. This is
  just wrong: `Failure _` should be reserved to fatal program errors,
  and should not be caught for control-flow.

- The current implement is buggy (it fails all the time, but we don't
  notice because no warning is emitted):
  CamlinternalFormat.open_box_of_string expects a string of the form
  "v 3", but check_open_box would pass a string of the form "<v 3>"
  (or an empty string), which is the payload of the format value. So
  the check always fails.

- The idea of the check is wrong: "@[<x>foo@]" is an incorrect format
  string to pass to Format (the box indication does not make sense),
  but it is a perfectly fine format string to pass to Printf, where it
  just prints "@[<x>foo@]" on the output. So we cannot complain to the
  user at type-checking time, when we don't know how the format string
  will be used, whether the boxes will be interpreted as actual boxes
  or string literals.

Note: Typos found with https://github.com/codespell-project/codespell

Here is the (semi-manual) command used to get (and correct) the typos:

$ codespell -i 3 -w --skip=".png,.gif,./ocaml/boot,./ocaml/.git,./ocaml/manual/styles,./ocaml/manual/manual/htmlman" -L minimise,instal,contructor,"o'caml",cristal,pres,clos,cmo,uint,iff,te,objext,nto,nd,mut,upto,larg,exten,leage,mthod,delte,tim,atleast,langage,hten,iwth,mke,contant,succint,methids,eles,valu,clas,modul,que,classe,missings,froms,defaut,correspondance,differents,configury,reachs,cas,approche,normale,dur,millon,amin,oje,transfert

This state argument was passed around, but never used, except
by the continuation.