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