You can tell the parts she wrote on certain BJ songs and....
SIA:
I wanna fill the room, when she's in it with you
Please don't forget me
Do I imagine it, or catch these moments
I know you got history
Britney:
But I'm your girlfriend, now I'm your girlfriend
Trying to be cool
I hope I'm paranoid, that I'm just seeing things
That I'm just insecure
Like she very clearly threw in some parts after it was all done in a camp and she didn't even bother to match the rhyme schemes
