In the sample implementation, ephemerons are not disjoint from regular pairs. This behaviour is inherited from the Chez Scheme parent implementation.

The spec language ‘An ephemeron is a record structure with two fields’ strongly implies they should be disjoint.
SRFI 124 is silent on this matter but I would take it to imply disjointness as well.

So the sample implementation should probably be fixed. (It should wrap a Chez ‘ephemeron pair’ in a single-field record type.)

Daphne