The existing SRFIs 124 and 246 do not say anything about which Scheme values are suitable as ephemeron keys and as elements of a guardian.
For example, in Chez Scheme, an ephemeron with a fixnum key is never broken; an ephemeron with a bignum key can. (This reflects the delicate behaviour of eq? on numbers in Scheme.) One possible solution is to disallow values which do not refer to locations as weak keys.
