Start by dealing (or not) with "dotted pair" notation up front. eg (a . b) (a b . c) etc. This confronts a whole bunch of issues directly and immediately.

Start by dealing (or not) with "dotted pair" notation up front. eg (a . b) (a b . c) etc. This confronts a whole bunch of issues directly and immediately. What is a list, really? How do I handle recursion in my parsing--implementation language (fast--but maybe not tail recursive and weak to cycles) or lisp (slower--but probably infinite and handles circular lists)? How do I deal with garbage (long parses kick up a lot of garbage)? Dealing with dotted pairs is the difference that means you understand implementing lisp rather than are just toying with it.