There are many built-in functions in LISP that are not logically required to be built in. They are there for convenience, and in some cases because they are faster that way. The three functions presented below are built-in functions that have definitions in terms of other more basic functions and special forms.
(LAMBDA (X Y)
(COND ((EQ X NIL) Y)
((ATOM X) (CONS X Y))
(T (CONS (CAR X) (APPEND (CDR X) Y))))))
This version of APPEND is slightly more general than the commonly found ...