% empty? is already defined.
/push {exch cons}.
/pop {uncons exch pop}.
[2 3 4 5 6] 1 push
= [1 2 3 4 5 6]
[1 2 3 4 5 6] pop
=[2 3 4 5 6]
[2 3 4 5 6] empty?
=false
[] empty?
=true