When did Snape invent his curses?

[greg_a126]

No: HPFGUIDX 133378

The thing that's been bugging me about this whole him inventing these 
curses thing, is how did James learn a non-verbal spell that Snape had 
created.  Remus says that the curse was quite the fad when they were in 
school.  So if Snape inveneted it, someone else would've had to have 
also created it independent of Snape, OR Snape didn't actually invent 
them, but he just wrote them down in his book.

grega126