; Three fields select a direct published-source reference, rather than bel.
((tab 'key) (source (tabref tab 'key)) before)
; Both evaluate the key expression before reading the live entry list.
((tab (do (xdr (cdr tab) '((key . after))) 'key))
 (source (tabref tab (do (xdr (cdr tab) '((key . after))) 'key))) after)
((tab 'key) (source (tabref tab 'key)) after)
; The source helper closes over its own lexical scope, not the caller's =.
((let = (fn (a b) nil) (tab 'key))
 (source (let = (fn (a b) nil) (tabref tab 'key))) after)
; Existing-entry locations use dynamically bound equality during key search.
((dyn = (fn (a b) t) (car (where (tab 'other))))
 (source (dyn = (fn (a b) t) (get 'other (cddr tab)))) (key . after))
; Location lookup must also read entries after evaluating a mutating key.
((car (where (tab (do (xdr (cdr tab) '((key . newest))) 'key))))
 (source (get (do (xdr (cdr tab) '((key . newest))) 'key) (cddr tab))) (key . newest))