Rebinding is not mutation. This seems pedantic but is an important distinction. None of the semantics of the runtime are changed. The data remains immutable. You probably know this. However, for the benefit of readers who may be less familiar: Erlang does not allow variables to be rebound, so it's somewhat typical for Erlang code like this:

  X1 = 8.
  X2 = X1 + 1.
  X3 = X2 * 302.

You cannot, say, do this:

  X1 = 8.
  X1 = X1 + 1.

This is because in Erlang (and in Elixir) the `=` is not just assignment, it is also the operator for pattern matching. This has implications that are too broad for this post, but the key point here is that it's attempting to see if the the left side and the right side "can match".

Whereas writing the same thing in Elixir would look like:

  x = 8
  x = x + 1
  x = x * 302

This is because Elixir allows `x` to be rebound, in effect changing what data `x` points to, but not mutating the underlying data itself. Under the hood Elixir rewrites each expression into something resembling the Erlang version.

The practical effect of this is that if you for example insert a process spawn somewhere in between any of the lines that references `x`, that process gets its own totally immutable version of the data that `x` points to at that point in time. This applies in both Erlang and Elixir, as data in both is completely immutable.