Yes, a Lisp program can potentially contain arbitrary lists. It can directly contain arbitary lists or other objects as quoted literals. It can use macros to

Yes, a Lisp program can potentially contain arbitrary lists. It can directly contain arbitary lists or other objects as quoted literals. It can use macros to create arbitrary syntax, which is then used. (In mainstream dialects, macros cannot turn an arbitrary list into a form; it has to start with a symbol, after which it can be anything.)