Takes the topmost item from the stack as a key and looks up its value in the database.
PumpkinDB> "hi" [ASSOC?] READ
EmptyStack error if there are less than two items on the stack
NoTransaction error if there's no current write transaction
UnknownKey error if there is no such key. See ASSOC? for mediating this problem
works : "hi" "there" 2DUP [ASSOC COMMIT] WRITE SWAP [RETR] READ EQUAL?. requires_txn : ["hi" RETR] TRY UNWRAP 0x08 EQUAL?. empty_stack : [RETR] TRY UNWRAP 0x04 EQUAL?.