Read Only Range in source code

Consider a domain specific language like this

event <name> happens <condition>

with an AST node like Event(name: string, condition: Expression) where possible domain specific insertions via auto completion or typing into the placeholder end up into something like ...

event hello-world happens when universe > 42

In this approach I would love that the editor user cannot modify the basic "keyword" template of event<name>happens <condition> but only modify the placeholders in the brackets. There most be however some way to wipe the whole template 😀. Recursive usage also, since when<expression> is again a template.

This is prior art in editors (at least JetBrains MPS is using this methodology) which to my little knowledge they call a projection.

If this is already supported, open/rejected or discussed and I missed it, my apologies in advance.

No files, tests, or entry points are identified in the issue. Begin by locating the protocol areas related to source ranges and editor edits, then review existing discussions about recursive read-only templates. Done would require an agreed protocol design and corresponding implementation and tests.