Adjacency Lists

Nowadays, RPLACA and RPLACD are less used.

Straight lists are more common.

Memory space is less of a concern.

(setq nodes '(a b c d))

(setq arc-lists '(

(a (b c))

(b (d))

(c (d))

(d ())

) )

(defun is-edge (x y)

(member y (second (assoc x arc-lists))) )

(is-edge 'a 'c) ; => (C) i.e., true.

(is-edge 'a 'd) ; => NIL