Here is the file:

```
a b
c d
d c
b a

```

The 1st line is mirrored with the 4th line and 2nd line is mirrored with the 3rd line. I just want to keep one of the mirror lines. That means I want to delete line number 3 and line number 4. This is just an example. What I want is to have a code, which figures out the mirror lines and keep only one of them.