[conistonwater]

I think that's a coding style decision, and shouldn't be part of language specification. I certainly don't feel about primes the way you do: having a function f and a function f' usually means they are sort of similar with some specific difference between them, definitely not "couldn't think of a better name". E.g., in haskell a prime often denotes a strict function, it's a better convention than having f and f_strict. Plus, how are you going to decide which unicode symbols should not be allowed? It can get a little random.

[kbp]

It's definitely a matter of taste, but I feel like settling for "similar except different somehow" is the same thing as "couldn't think of a better name". I would much prefer f-strict because then I don't need to be familiar with the punctuation conventions of the codebase to know how it's different from f, it's right there in English.

Edit: Plus, then you don't have to wonder if that particular function follows that naming convention; prime meaning strict isn't nearly as universally followed as ? or ! in Scheme, for instance.