nconc

(nconc &rest lists)

Destructively concatenates its list arguments by setting the last cdr of each non-empty list to point at the following argument, then returns the first non-nil argument. No new cons cells are allocated, so the argument lists are modified in place. (nconc) returns nil, (nconc x) returns x, and nil arguments are skipped. The last argument may be any object (it is spliced onto the tail of the preceding list without being copied).