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

# copy all lines of input, prefixed by its number, kind
# of `cat -n'

# switch to buffer
x

# init the counting
1{
        s/^/0/
}

# increment the count: first line == number 1
: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

# format the number like printf's `"%6d"'
s/^/      /
s/^.*\(......\)/\1/

# append the line to the number, and write: "<number>  <line>"
# note: this is the format of gnu-cat 
G
s/\n/  /p

# after printing the line, transform the line into the number, and
# store it on buffer again
s/^ *//
s/  .*//
h