Signed-off-by: Jim Pryor <profjim@jimpryor.net>
A third way to think is to identify the lambda formula not with classes of alphabetic sequences, but rather with abstract structures that we might draw like this:
-<pre><code>λ
- ... ___ ...
-^ |
-|______|
+<pre><code>
+ λ ... ___ ...
+ ^ |
+ |______|
</code></pre>
Here there are no bound variables, but there are *bound positions*. We can regard formula like (a) and (b) as just helpfully readable ways to designate these abstract structures.