s/^/k0/
tn
:n
s/k10201//
t
h
s/1/ hello/g
s/2/0!/g
s/0/ world/g
s/k *//
p
g
s/$/+/
ti
:i
s/0+/1/
s/1+/2/
s/k+/k1/
tn
s/2+/+0/
ti