The author might be new to Lisp, and I hope what I write doesn't discourage him. There are some other mistakes:

     ((ab . c) . d . nil)

isn't a valid s-expression. Maybe it should be

    ((ab . c) d . nil)

Also,

    (eq '(a b) '(a b))
    ; (a b) is a list and cannot be evaluated by eq

eq works fine on lists. It returns T iff its arguments are the same object (i.e. are at the same memory location, or are small enough integers or floats). If they're copies (as is the case in the above), it returns NIL. If this isn't what you expect, you should probably be using equal.