class stack begin ref(stack member) first ref(stack member) procedure top top: -first procedure pop if -empty then first: first next member procedure push(e); ref (stack member)e begin if first =/=none then e next member -first first: -e end push boolean procedure empty empty: -first==none first: -none end stackclass stack; begin ref (stack_member) first; ref (stack_member) procedure top; top:-first; procedure pop; if ¬empty then first:-first.next_member; procedure push(e);ref (stack_member) e; begin if first =/=none then e.next_member:-first; first:-e end push; boolean procedure empty; empty:=first= =none; first:-none end stack