I'm rereading HP books and just come upon Snape's grudge chapter. My question is why did Snape called Lupin to figure out if the old paper had dark magic or not. I mean Snape is the one who is expert at dark magic. I know meta wise it was a way to save Harry's behind and for Lupin to end up with the map for the third act but it doesn't make sense to me that Snape would call Lupin. If anything, he would go to Dumbledore.