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