generalize Linkage, add relevant comments

graph/iterator.go

@@ -30,7 +30,14 @@ type Tagger struct {
 	fixedTags map[string]Value
 }
 
-type LinkageSet struct {
+// TODO(barakmich): Linkage is general enough that there are places we take
+//the combined arguments `quad.Direction, graph.Value` that it may be worth
+//converting these into Linkages. If nothing else, future indexed iterators may
+//benefit from the shared representation
+
+// Linkage is a union type representing a set of values established for a given
+// quad direction.
+type Linkage struct {
 	Dir    quad.Direction
 	Values []Value
 }