Say I have the following kind of file:

```
a a c
b b c
c c c
d d c
e e c
a a c
b b c
c c c
d d c
e e c

```

How do I end up with:

```
a a c
b b c
c c c
—————
d d c
e e c
a a c
b b c
c c c
d d c
e e c

```

...without just adding the `em dashes` after the 3rd line (first line with `c c c`).