Another prerequisite is algebra: if one does not understand that substituting "X" in "X²" by "A+1" makes "(A+1)²", how are they supposed to substitute parts of program with function calls?

This alone is enough to get my upvote. I often struggle to explain that programming is a form of math (or at least, that it needs math). One typical answer goes like

When you are designing algorithms, sure, but these days we're mostly plugging libraries together.

My suffocation and stuttering (refusing to change one's mind in the face of compelling sounding arguments tends to do that) squash any attempt at a proper rebuttal. But now, I have one:

Using function calls? I hope for your sake that you get that pesky mathematical concept of "substitution".