#!/usr/bin/sed -nf
# Carlos J.Duarte

# copy all lines of input, prefixing only non blank lines by its number, 
# kind of `cat -b'

# init counter
1{
        x
        s/^/0/
        x
}

# for blanks, don't incr count, but print
/./!{
        p
        b
}

# for the rest is the same as a `cat -n' 
x
:d
s/9\(_*\)$/_\1/
td
s/^\(_*\)$/0\1/
s/8\(_*\)$/9\1/
s/7\(_*\)$/8\1/
s/6\(_*\)$/7\1/
s/5\(_*\)$/6\1/
s/4\(_*\)$/5\1/
s/3\(_*\)$/4\1/
s/2\(_*\)$/3\1/
s/1\(_*\)$/2\1/
s/0\(_*\)$/1\1/
s/_/0/g

s/^/      /
s/^.*\(......\)/\1/

G
s/\n/  /p
s/^ *//
s/  .*//
h