User blog comment:Ubersketch/CNF in my FSS definition/@comment-35470197-20190808221731/@comment-35470197-20190809003908

I guess that why you could not understand the problem is because
 * 1) You are comfounding \(f\) and \(f |_X\) for a function \(f\) whose domain contains a set \(X\). In order to avoid this misunderstanding, eclare the domain.
 * 2) You are comfounding the substituted term \(a\) in the expression \(f(a)\) with the indeterminate \(a\) for a function \(f(a)\). In order to avoid this misunderstanding, quantify the term \(a\) in order to disctinguish it from the indeterminate.