h
s/\(.\).*/&\1/p
s/./ /g
G
G
:
/.\(.*\).\n.\(.\)\(.*\n.*\)\(.\)/{s//\4\1\2\n.\3/
P}
t
s/.*\n//
G
:0
s/\(\n.*\)\(.\)/\2\1/
t0