We did add better immutable-system support thanks to Dave Cooper,
is this meant to be the final public API (an exported global variable that holds a hashtable)?
The final API is (register-immutable-system "foo") and, I suppose, (sysdef-immutable-system-search "foo") though I forgot to export said function... fixed.
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.
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