Change filetype sl to renderman shader language instead of slang

Vim detects the extension .sl as slang filetype. I want that vim
detects the extension .sl as renderman shader lang filetype by
default and not as slang filetype.

I have add to my .vimrc:
au BufRead,BufNewFile *.sl setf sl

But it doesn't work. Do you know any solution?

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