What is a valid name for a function?

Examples

-- works
let µ x = x * x  
let ö x = x * x

-- doesn't work  
let € x = x * x  
let § x = x * x

I am not sure, but my hunch is that Haskell doesn't allow Unicode function names, does it? (Unicode like in http://www.cse.chalmers.se/~nad/listings/lib-0.4/Data.List.html)

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