+Note [Growing the tau-tvs using constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+(grow preds tvs) is the result of extend the set of tyvars tvs
+ using all conceivable links from pred
+
+E.g. tvs = {a}, preds = {H [a] b, K (b,Int) c, Eq e}
+Then grow precs tvs = {a,b,c}
+
+All the type variables from an implicit parameter are added, whether or
+not they are mentioned in tvs; see Note [Implicit parameters and ambiguity]
+in TcSimplify.
+
+See also Note [Ambiguity] in TcSimplify
+