Write a "code walker" (an R function itself) that takes an
argument 'expr', which is an R expression (think of a *body* of a
function 'f') and is able to mark e.g. warn about such
"dead" code in 'expr' (and hence in function 'f' ...) --- with
the important underlying assumption that all functions that are
called inside 'f' have "no side effects".