In general I share your increasing uncertainty, but I think it is safe to say that understanding a program is made much more difficult once one has the possibil

In general I share your increasing uncertainty, but I think it is safe to say that understanding a program is made much more difficult once one has the possibility that a variable might be modified outside of the scope it is created for. The idiom of using mutable variables to track the current state of affairs in a loop is harmless up to the point where one or more is passed by mutable reference to a subroutine or impure function; at that point, it has, almost literally, fallen down a rabbit-hole wherein almost anything could happen.