+mapped onto 2. What does your evaluator code do?
+
+We'll guide you to a solution involving closures. The first step is
+to allow values to carry around a specific environment with them:
+
+ type value = LiteralV of literal | Closure of lambdaTerm * env
+
+This will provide the extra information we need to evaluate an
+identifier all the way down to the correct final result. Here is a
+[[modified version of the evaluator that provides all the scaffoling for
+passing around closures|exercises/reduction_with_closures]].
+The problem is with the following line:
+
+ | Closure (Abstract(bound_ident, body), saved_r) -> eval body (push bound_ident arg saved_r) (* FIX ME *)
+
+What should it be in order to solve the problem?
+
+
+## Monads
+
+Mappables (functors), MapNables (applicatives functors), and Monads
+(composables) are ways of lifting computations from unboxed types into
+boxed types. Here, a "boxed type" is a type function with one missing
+piece, which we can think of as a function from a type to a type.
+Call this type function M, and let P, Q, R, and S be variables over types.
+
+Recall that a monad requires a singleton function 1:P-> MP, and a
+composition operator >=>: (P->MQ) -> (Q->MR) -> (R->MS) that obey the
+following laws:
+
+ 1 >=> k = k
+ k >=> 1 = k
+ j >=> (k >=> l) = (j >=> k) >=> l