+What alternatives did I consider? Annotating the loop itself doesn't
+work because (a) it is local and (b) it will be w/w'ed and I having
+w/w propagating annotation somehow doesn't seem like a good idea. The
+types of the loop arguments really seem to be the most persistent
+thing.
+
+Annotating the types that make up the loop state s doesn't work,
+either, because (a) it would prevent us from using types like Either
+or tuples here, (b) we don't want to restrict the set of types that
+can be used in Stream states and (c) some types are fixed by the user
+(e.g., the accumulator here) but we still want to specialise as much
+as possible.
+