It's often said that dynamic systems are distinguished because they are the ones in which **order matters**. However, there are many ways in which order can matter. If we have a trivalent boolean system, for example---easily had in a purely functional calculus---we might choose to give a truth-table like this for "and":
-<pre><code>
-true and true = true
-true and true = true
-true and * = *
-true and false = false
-* and true = *
-* and * = *
-* and false = *
-false and true = false
-false and * = false
-false and false = false
-</code></pre>
+ true and true = true
+ true and true = true
+ true and * = *
+ true and false = false
+ * and true = *
+ * and * = *
+ * and false = *
+ false and true = false
+ false and * = false
+ false and false = false
And then we'd notice that `* and false` has a different intepretation than `false and *`. (The same phenomenon is already present with the material conditional in bivalent logics; but seeing that a non-symmetric semantics for `and` is available even for functional languages is instructive.)
and there's no more mutation going on there than there is in:
- <!--
- <pre>
- <code>∀x. (F x or ∀x (not (F x)))</code>
- </pre>
- -->
+ <pre><code>∀x. (F x or ∀x (not (F x)))
+ </code></pre>
When a previously-bound variable is rebound in the way we see here, that's called **shadowing**: the outer binding is shadowed during the scope of the inner binding.