Write closing thoughts in development log

LOG.md

@@ -238,3 +238,38 @@ The final pipeline I ended up with was:
 
 I then created an `evaluator` function to wrap this up in the `eval`
 module, to allow for a simple interface to the whole library.
+
+## Conclusion / Closing thoughts
+
+Creating the differentiation program was a lot of fun, and I'm happy
+with the result I ended up with.  The code is well-tested, and for the
+most part is fairly maintainable (perhaps with the area needing the
+most work in that respect being the constant folding pass).
+
+In order to quantify how effective the test suite is, I ran a mutation
+testing tool (`cargo-mutants`) and it found that only 3 out of 221
+generated mutants were missed by the test suite, giving a mutation
+score of 98.6% which is very good.  The three missed mutants were also
+limited to the demo program itself (which makes sense since it has no
+tests) and error message formatting code, so none of them are
+particularly concerning.
+
+There are quite a lot of areas where further work could be done, such
+as adding support for functions like sine, cosine and natural
+logarithms; enhancing the simplification and canonicalisation passes
+and allowing non-constant exponents.  However, I think the most
+interesting enhancement would be to add equalities to the language and
+allow definitions to be stored within a session, with explicit
+differentiation commands to get the deriviative of something, such as:
+
+```
+> y = x^3 - x^2
+> dy/dx
+3x^2 - 2x
+```
+
+This, along with a few other features, would transform the program
+into a more general-purpose symbolic mathematics engine, which would
+be very cool and useful.  The current architecture would allow for
+this quite nicely, as evaluators are able to have mutable state which
+could be used for storing variable bindings.