Yes, brackets, but why is this a problem?

I found this code [1] in GHC which seems like it might be relevant. Is it? It seems to suggest that classes Pc, Pd, Po, Sm, Sc, Sk and So are allowed in infix names, but Ps, Pe, Pi and Pf are not. Am I reading that right?

[1] https://github.com/ghc/ghc/blob/49012ebc9ed44a0b1f8de3781e15c8115d3074f8/compiler/parser/Lexer.x#L2013

Cheers,

David