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+
+Binding more than one variable at a time
+----------------------------------------
+
+It requires some cleverness to use the link monad to bind more than
+one variable at a time.  Whereas in the standard Reader monad a single
+environment can record any number of variable assignments, because
+Jacobson's monad only tracks a single dependency, binding more than
+one pronoun requires layering the monad.  (Jacobson provides some
+special combinators, but we can make do with the ingredients already
+defined.)
+
+Let's take the following sentence as our target, with the obvious
+binding relationships:
+
+<pre>
+    John believes Mary said he thinks she's cute.
+     |             |         |         |
+     |             |---------|---------|
+     |                       |
+     |-----------------------|
+</pre>
+
+It will be convenient to
+have a counterpart to the lift operation that combines a monadic
+functor with a non-monadic argument:
+
+<pre>
+    let g f v = ap (unit f) v;;
+    let g2 u a = ap u (unit a);;
+</pre>
+
+As a first step, we'll bind "she" by "Mary":
+
+<pre>
+believes (z said (g2 (g thinks (g cute she)) she) mary) john
+
+~~> believes (said (thinks (cute mary) he) mary) john
+</pre>
+
+As usual, there is a trail of *g*'s leading from the pronoun up to the
+*z*.  Next, we build a trail from the other pronoun ("he") to its
+binder ("John").  
+
+<pre>
+believes
+  said 
+    thinks (cute she) he
+    Mary
+  John
+
+believes
+  z said
+    (g2 ((g thinks) (g cute she))) he
+    Mary
+  John
+
+z believes
+  (g2 (g (z said)
+         (g (g2 ((g thinks) (g cute she))) 
+            he))
+      Mary)
+  John
+</pre>
+
+In the first interation, we build a chain of *g*'s and *g2*'s from the
+pronoun to be bound ("she") out to the level of the first argument of
+*said*. 
+
+In the second iteration, we treat the entire structure as ordinary
+functions and arguments, without "seeing" the monadic region.  Once
+again, we build a chain of *g*'s and *g2*'s from the currently
+targeted pronoun ("he") out to the level of the first argument of
+*believes*.  (The new *g*'s and *g2*'s are the three leftmost).
+
+<pre>
+z believes (g2 (g (z said) (g (g2 ((g thinks) (g cute she))) he)) mary) john
+
+~~> believes (said (thinks (cute mary) john) mary) john
+</pre>
+
+Obviously, we can repeat this strategy for any configuration of pronouns
+and binders.
+