Incorporate consensus feedback.

Author: Luke Sneeringer

aip/general/0159/aip.md

@@ -6,9 +6,39 @@ what collection it is in.
 
 ## Guidance
 
-APIs **may** support reading resources across multiple collections by allowing
-users to specify a `-` (the hyphen or dash character) as a wildcard character
-in a standard [`List`][aip-132] operation:
+There are two potential approaches for reading resources across multiple
+collections. Groups of related services **should**, as a unit, select a
+preferred approach and apply it consistently.
+
+### Promoting Resources
+
+One approach is to evade the problem entirely by promoting resources above
+their erstwhile parents in the event that reading across collections is likely
+to be a requirement.
+
+In this approach, the parent becomes a field on the resource:
+
+```typescript
+interface Book {
+  // The ID of the book's publisher
+  publisherId: string;
+
+  // Other fields...
+}
+```
+
+Similarly, the parent is removed from the URI:
+
+```
+GET /v1/books?filter...
+```
+
+### Wildcards
+
+The other approach is to use a wildcard character. Services **may** support
+reading resources across multiple collections by allowing users to specify a
+`-` (the hyphen or dash character) as a wildcard character in a standard
+[`List`][aip-132] operation:
 
 ```
 GET /v1/publishers/-/books?filter=...