Description

Here we define 'block' as a set of consecutive lines which contain some string, for example '<OPTION>'.
Now we want to add a line '---End of options---' to the end of every block.

Raw Input

The main Linux distributions are:
 <OPTION> Red Hat
 <OPTION> Mandrake
  <OPTION> SuSE
   <OPTION> Slackware
Main window manager used in Linux are:
   <OPTION> Sawfish
  <OPTION> enlightenment
   <OPTION> fvwm

Desired Output

The main Linux distributions are:
 <OPTION> Red Hat
 <OPTION> Mandrake
  <OPTION> SuSE
   <OPTION> Slackware
---End of options---
Main window manager used in Linux are:
   <OPTION> Sawfish
  <OPTION> enlightenment
   <OPTION> fvwm
---End of options---

Script and Comments

Script₁

[ 1] /<OPTION>/{
[ 2] $!N
[ 3] P
[ 4] /\n.*<OPTION>/!i\
[ 5] ---End of options---
[ 6] D
[ 7] }

Comments

If a line does not contain \ at all, just print it out and start next cycle.
If a line contains \, Step [2] will join next line if exists.

After Step [2], Pattern Space may contains

Pattern Space (... stands for any text)	description	actions to take
...\...	only one line	i
...\...\n .....	the joined line does not contain \	P,i,D
...\....\n...\...	the joined line contains \	P,D