Insert a line into text file

This is really annoying me.
Should be simple. Basically, I'm trying to insert a line to a test file...

myfile:TStream=WriteStream("test.txt")
	If WriteLine(myfile,"Hello") Print "Hello successful!"
	If WriteLine(myfile,"World") Print "World successful!"
CloseStream(myfile)
myfile1:TStream=OpenStream("test.txt")
ReadLine(myfile1)
If WriteLine(myfile1,"AGAIN") Print "AGAIN successful"
CloseStream(myfile1)
myfile2:TStream=ReadStream("test.txt")
	While Not Eof(myfile2)
   		Print ReadLine(myfile2)
	Wend
CloseStream(myfile2)

<edit> P.s. tried simple writefile/openfile/readfile. I just can't get Openstream/file to read and write. Seems to only allow one of them.

Tonyg,
Have you tried looking at the seekstream, streampos functions?

I have, and I have just found one of my own posts suggesting it to somebody else.
I may well be trying to forget about this post in a few minutes but I still won't know why I need to seekstream.
<edit> Hmmm, it's still a case of inserting a line.
My guess is I will have to read and write each line.
Maybe I did consider seekstream. It will only allow me to write to a certain position in the file. However, if that position is occupied it will overwrte that line.
Without rewriting the while file I can't see how to insert.
(which makes sense of course).