In ocamltest tests, it would be useful to have a concept of test blocks that are triggered upon failure of a previous action. For instance, you can currently trigger a test only on macOS using:

macos;
{
  ...
}

However, to trigger a test on non-macOS configs, you would need to modify ocamltest to add a new “builtin action” not-macos, which doesn’t exist at time of writing. But maybe we could do something like:

macos;
{
  (* Executed on macOS *)
}
!{
  (* Executed on non-macOS *)
}

Conceptually, both { } and !{ } denote chlidren nodes, but !{ } nodes would be triggered upon failure of the parent action.

Having this would allow to avoid having both windows and not-windows in the “bulitin actions”.

Let me know if I’m missing something.