+
+Note [Extending the TvSubst]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The following invariant should hold of a TvSubst
+
+ The in-scope set is needed *only* to
+ guide the generation of fresh uniques
+
+ In particular, the *kind* of the type variables in
+ the in-scope set is not relevant
+
+This invariant allows a short-cut when the TvSubstEnv is empty:
+if the TvSubstEnv is empty --- i.e. (isEmptyTvSubt subst) holds ---
+then (substTy subst ty) does nothing.
+
+For example, consider:
+ (/\a. /\b:(a~Int). ...b..) Int
+We substitute Int for 'a'. The Unique of 'b' does not change, but
+nevertheless we add 'b' to the TvSubstEnv, because b's type does change
+
+This invariant has several crucial consequences:
+
+* In substTyVarBndr, we need extend the TvSubstEnv
+ - if the unique has changed
+ - or if the kind has changed
+
+* In substTyVar, we do not need to consult the in-scope set;
+ the TvSubstEnv is enough
+
+* In substTy, substTheta, we can short-circuit when the TvSubstEnv is empty
+
+