Most designers set requirements for f() by describing what f() should be, which is a circularity. To be useful, requirements should be defined independent of f() as tests for fitness.
The computer can look at every word on the page, every phrase, name, quote, and section of text, and show me a "map" of the words and ideas behind which lay the most interesting ideas I might want to know about. Links are no longer lonesome strands precariously holding together a sparsely connected Web, but a booming choir of ephemeral connections... See more
The product designer's task is to create a new f(). The designer doesn't get to define x: that's empirical. And they don't get to dictate y either. A given y is only a worthwhile target if it's worth paying for in the eyes of the user — also empirical. That means x and y are requirements for f(). They are fixed, f() is variable.