feat(schema): add narrows edge

kythe/docs/schema/schema.txt

@@ -838,6 +838,62 @@ though the edge A' *generates* B were produced. This edge is useful in cases
 where it isn't otherwise possible to produce the VName for A' and where the
 span A in source text can uniquely identify A' by the *defines/binding* edge.
 
+[[narrows]]
+narrows
+~~~~~~~
+
+Brief description::
+  A *narrows* B if A identifies the same target language object as B but is
+  subject to additional constraints.
+Commonly arises from::
+  type refinement
+Points from::
+  variables
+Points toward::
+  variables
+Ordinals are used::
+  never
+See also::
+  <<generates>>, <<typed>>
+
+Some languages allow the types of variables to change over the course of a
+program. For example, a type test can be used to narrow the type of a variable:
+
+[source,java]
+----
+Object x;  // x : Object
+if (x instanceof String) {
+  x;  // x : String
+}
+----
+
+In this example, every `x` refers to the same variable `x`, but the `x` in
+the body of the conditional has a different static type than both the `x`
+in the conditional expression and the `x` declared at the top. This program
+is represented in the following way:
+
+[source,java]
+----
+//- @x defines/binding XObject
+//- XObject typed Object
+Object x;
+//- @x ref XObject
+if (x instanceof String) {
+  //- @x ref XString
+  //- XString narrows XObject
+  //- XString typed String
+  x;
+}
+----
+
+The `x` with type `String` has no `defines/binding` edge, but is associated
+with the `x` with type `Object` with the `narrows` edge. Clients should
+interpret these variables as the same (in the way that nodes associated with
+the `generates` edge are the same), but should prefer facts and edges from the
+particular `x` being identified. In particular, `documents` edges and `code`
+facts should be chosen from the particular node that is the target of a `ref`
+when that node is in a `narrows` relationship.
+
 [[named]]
 named
 ~~~~~