:0
N
$!b0
s/^/#/mg
:
s/#\(.\)\|\(#\)/\1\2#/mg
s/##/ #/g
/\S#\|^ #/b
s/^/# /mg
h
s/\n.*//
s/[^#]/ /g
H
G
h
s/\n.*//
s/./#/g
H
G