A distinct and immutable pair is... A pair. (eq? (pair? '(A . B)) #t) (eq? (pair? (list 1 2 3)) #f) But agree you probably want list-ref instead of c

A distinct and immutable pair is... A pair. (eq? (pair? '(A . B)) #t) (eq? (pair? (list 1 2 3)) #f) But agree you probably want list-ref instead of cddr and friends.