append!

(append! list ...)

Appends the lists destructively: the last pair of each non-empty list is changed to point at the next list, and the result shares structure with its arguments. Use it only on lists you built yourself, never on a quoted literal. A Structure and Interpretation of Computer Programs (SICP)/MIT name, not R7RS: no library exports it, so it is visible only to a file with no import and at the REPL.