################## 
#
#  Pair  (Lisp S-Node)

        # A lisp "Pair" contains a left and a right.
        # Left may be anything, but right must always be another Pair, or null.

        p7class Pair { (tcl)left (Pair)right }

        # "cons" allocates a new Pair with its two parameters

        p7proc (Pair)cons { (tcl)a (Pair)b } {
            set (Pair)z [new (Pair)]
            set z(left) $a
            set z(right) $b
            set z
        }

        # "hd" gets the left slot
        p7proc (tcl)hd { (Pair)x } { k $x(left) }

        # "tl" gets the right slot
        p7proc (Pair)tl { (Pair)x } { k $x(right) }