The look on some of the attendees' faces seemed to suggest I was being a wise-guy. I wasn't trying to be snarky; I expected his response because I'd heard it repeatedly and found through personal investigation of engineering curricula that it is not in the mainstream.
Substitution of terms in expressions is a fundamental concept. It is so fundamental that kids who are allowed to handle money will do it intuitively when making change. It is bizarre that such a fundamental hypothesis of calculation in computer science and mathematical logic is elided in modern computer science degree programs.
(The same kids will run into difficulty reframing their experience, when given non-monetary challenges in math classes. They get counting and computing when it is informed by their use of language; they lose the cognition when the language is removed. So add the vocabulary and operators of substitution to their repertoire , early, often, and concretely, and perhaps they will actually start intuiting what computation is all about.)