[thaumasiotes]

No, that's irrelevant.

Lisp will treat (a - b) as 5 tokens, just the same way it will treat (- a b) as 5 tokens. Infix operators are completely unrelated to this problem. What lisp is doing is determining tokens by reference to spacing (the parentheses don't need to be spaced; I believe they are reader macros but in any event they are special-cased) and then acting on the tokens. What C is doing is not that; the concept is that you eliminate all spacing before you decide what the tokens are.

So in C, there is no such thing as "a - b", only "a-b", and that's why "a-b" cannot be used as an identifier.

If you want to write your lisp in infix notation, you can, but it will remain true that (a - b) is a list with 3 elements and (a-b) is a list with 1 element, which is what matters here.