Interned and unreadable symbols are only weakly held by
the internal symbol table. This weakness can never
affect the result of an eq?, eqv?, or equal? test, but a
symbol may disappear when placed into a weak box (see
Weak Boxes) used as the key in a weak hash table (see
Hash Tables), or used as an ephemeron key (see Ephemerons).
Kinda. It irks me that a value that's still arguably reachable (via 'read) could be observably lost by a weak box, weak hash table, or ephemeron. But it seems feasible to create readable-counts-as-reachable versions of these data structures on top of the built-in ones, so the built-in ones expose strictly better lower-level control.
