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.ml]]. What you need to do is
+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.