Given two Turing machines $M,M'$, is it possible to check whether $M$ invokes $M'$?
In other words, is the following problem computable/decidable?
Inputs: Turing machines $M,M'$
Question: Does $M$ ever invoke $M'$?
Perhaps we can look whether $M$ includes the code of $M'$ in it? In a C program we can check whether a function
f invokes a function
g, right? Can we do this for Turing machines too?