#!/bin/sed -f

# Put 80 spaces in the buffer
1 {
x
s/^$/          /
s/^.*$/&&&&&&&&/
x
}

# del leading and trailing spaces
y/\t/ /
s/^ *//
s/ *$//

# add a newline and 80 spaces to end of line
G

# keep first 81 chars (80 + a newline)
s/^\(.\{81\}\).*$/\1/

# \2 matches half of the spaces, which are moved to the beginning
s/^\(.*\)\n\(.*\)\2/\2\1/