Rem append a.txt and b.txt onto the end of file d.txt
Rem leaving a.txt and b.txt as they were.
copy d.txt + a.txt + b.txt  d.txt

Rem there is a abbreviated way of writing that.
Rem I don't recommended it. I tell you about it only so you will won't be puzzled when you see it.
copy d.txt + a.txt + b.txt