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:
```    John believes Mary said he thinks she's cute.
|             |         |         |
|             |---------|---------|
|                       |
|-----------------------|
```
It will be convenient to have a counterpart to the lift operation that combines a monadic functor with a non-monadic argument:
```    let g f v = ap (unit f) v;;
let g2 u a = ap u (unit a);;
```
As a first step, we'll bind "she" by "Mary":
```believes (z said (g2 (g thinks (g cute she)) she) mary) john

~~> believes (said (thinks (cute mary) he) mary) john
```
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").
```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
```
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).
```z believes (g2 (g (z said) (g (g2 ((g thinks) (g cute she))) he)) mary) john

~~> believes (said (thinks (cute mary) john) mary) john
```
Obviously, we can repeat this strategy for any configuration of pronouns and binders.