6. A snag: what happens when we want to replace a variable with a term
that itself contains a free variable?
- term environment
- ------------- -------------
- (\w.(\y.y)w)2 []
- (\y.y)w [w->2]
- y [w->2, y->w]
+<pre>
+term environment
+------------- -------------
+(\w.(\y.y)w)2 []
+(\y.y)w [w->2]
+y [w->2, y->w]
+</pre>
In the first step, we bind `w` to the argument `2`. In the second
step, we bind `y` to the argument `w`. In the third step, we would