5.531 Therefore I write not “f(a, b) .a = b” but rather “f(a, a)” (or “f(b, b)”). And not “f(a, b) . ~a = b”, but rather “f(a, b)”.

No need to use the identity sign, in other words. Just the same sign for the same thing always. But what would “f(a, a)” mean? What would be the point?