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.