TODO: Register
TODO: Variable vs binder
TODO: simplification -> Normalisation?
+TODO: Use saturated (application) instead of complete (application)?
+TODO: core => Core
+TODO: \defref{beta-reduction} -> \defref{β-reduction}
+TODO: Make interaction links not bold
+TODO: Say something about implementation differences with transformation specs
+TODO: Say something about the builtin functions somewhere (ref: christiaan)