concatenate zero or more lists
Major Section:  ACL2-BUILT-INS

Append, which takes zero or more arguments, expects all the arguments except perhaps the last to be true (null-terminated) lists. It returns the result of concatenating all the elements of all the given lists into a single list. Actually, in ACL2 append is a macro that expands into calls of the binary function binary-append if there are at least two arguments; if there is just one argument then the expansion is that argument; and finally, (append) expands to nil.

Append is a Common Lisp function. See any Common Lisp documentation for more information.