clear
add z
print
level 0: z
add x
print
level 0: z
level 1: x
add y
print
level 0: y
level 1: x z
add w
print
level 0: y
level 1: x z
level 2: w
add v
print
level 0: y
level 1: w z
level 2: v x
add u
print
level 0: w
level 1: v y
level 2: u z
add t
print
level 0: w
level 1: u y
level 2: t v z