To give a concrete example: Haskell supports something called do-notation. You cannot "replace" parts of it, because it is syntactic sugar that will turn into a

To give a concrete example: Haskell supports something called do-notation. You cannot "replace" parts of it, because it is syntactic sugar that will turn into a number of expressions. But all of those expressions are then RT. In the same way you can have a C++ program that is FP (completely RT) but uses templates, which are obviously not "replacable". I hope that makes it more clear.