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index 8d68dda..3fac64b 100644
--- a/README.markdown
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@@ -2,7 +2,6 @@
 
 ![NICTA](http://i.imgur.com/sMXB9XB.jpg)
 
-
 ### Abstract
 
 Let's Lens presents a series of exercises, in a similar format to
@@ -21,6 +20,9 @@ best to start at the `Lets.GetSetLens` module. If you wish to derive the
 structure of lenses from first principles, then derive the more modern theories,
 start at the `Lets.Lens.Lens` module.
 
+Exercises can be recognised by filling in a function body that has a placeholder
+of `error "todo: <function-name>"`.
+
 ----
 
 ### Exercise modules