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diff --git a/exercises/_assignment6.mdwn b/exercises/_assignment6.mdwn
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--- a/exercises/_assignment6.mdwn
+++ b/exercises/_assignment6.mdwn
@@ -24,5 +24,61 @@ recursive reduction function.)
 
 <!-- just add early no-op cases for Ka and Sab -->
 
-## Evaluation in the untyped lambda calculus
+## Evaluation in the untyped lambda calculus: substitution
+
+Once you grok reduction and evaluation order in Combinatory Logic,
+we're going to begin to construct an evaluator for a simple language
+that includes lambda abstraction.  We're going to work through the
+issues twice: once with a function that does substitution in the
+obvious way.  You'll see it's somewhat complicated.  The complications
+come from the need to worry about variable capture.  (Seeing these
+complications should give you an inkling of why we presented the
+evaluation order discussion using Combinatory Logic, since we don't
+need to worry about variables in CL.)  
+
+We're not going to ask you to write the entire program yourself.
+Instead, we're going to give you [[the complete program, minus a few
+little bits of glue|code/reduction_with_substitution.ml]].  What you need to do is
+understand how it all fits together.  When you do, you'll understand
+how to add the last little bits to make functioning program.  
+
+1. In the previous homework, you built a function that took an
+identifier and a lambda term and returned a boolean representing
+whether that identifier occured free inside of the term.  Your first
+task is to complete the `free_in` function, which has been crippled in
+the code base (look for lines that say `COMPLETE THIS LINE`).  Once
+you have your function working, you should be able to run queries such
+as this:
+
+    # free_in "x" (App (Abstract ("x", Var "x"), Var "x"));;
+    - : bool = true
+
+2. Once you get the `free_in` function working, you'll need to
+complete the `substitute` function.  You'll see a new wrinkle on
+OCaml's pattern-matching construction: `| PATTERN when x = 2 ->
+RESULT`.  This means that a match with PATTERN is only triggered if
+the boolean condition in the `when` clause evaluates to true.
+Sample target:
+
+    # substitute (App (Abstract ("x", ((App (Abstract ("x", Var "x"), Var "y")))), Constant (Num 3))) "y" (Constant (Num 4));;
+    - : lambdaTerm = App (Abstract ("x", App (Abstract ("x", Var "x"), Constant (Num 4))), Constant (Num 3))
+
+3. Once you have completed the previous two problems, you'll have a
+complete evaluation program. Here's a simple sanity check for when you
+get it working:
+
+    # reduce (App (Abstract ("x", Var "x"), Constant (Num 3)));;
+    - : lambdaTerm = Constant (Num 3)
+
+What kind of evaluation strategy does this evaluator use?  In
+particular, what are the answers to the three questions about
+evaluation strategy as given in the discussion of [[evaluation
+strategies|topics/week3_evaluation_order]] as Q1, Q2, and Q3?
+
+## Evaluation in the untyped calculus: environments
+
+Ok, the previous strategy sucked: tracking free and bound variables,
+computing fresh variables, it's all super complicated.
+Here's a better strategy.
+