maybe a simple defun SYSTEM-MUTABLE-P and a setf variant would be better?
Maybe. I'll let Robert decide if he wants a way to make a system mutable no more. Up until now, the usage scenario was that systems would transition one way only from mutable to immutable, as you prepare an image for delivery.
I'm reluctant to add more code unless there's a demonstrated need for the functionality. I'd remind everyone that ASDF has ballooned in size and for all our efforts that inevitably means it has collapsed in maintainability!
If you'd like an extension to provide either introspection or "mutabilification", please file an enhancement ticket. But unless you provide a use-case with it, it is likely to get short shrift.
no, i'm fine with the functinality of REGISTER-IMMUTABLE-SYSTEM.
what i wasn't fine with is an exported global holding a hashtable, and at that time i hadn't noticed RIS because i was expecting a different name. REGISTER-IMMUTABLE-SYSTEM doesn't register any systems, it rather marks a system, that is already known by asdf, immutable. it's a different story that the implementation registers it in a hashtable as opposed to e.g. setting a flag on the system object.
so, before i knew about RIS, i proposed an API that i still think would be better, namely SYSTEM-MUTABLE-P.
if it's still feasible i suggest to replace REGISTER-IMMUTABLE-SYSTEM with (SETF SYSTEM-MUTABLE-P) and stop exporting *IMMUTABLE-SYSTEMS*.