Finish fleshing out Model; refactor mixin types; TRUNCATE

lib/src/collection/AsyncCollection.ts

@@ -496,7 +496,9 @@ export class AsyncCollection<T> {
 
             if ( !fetched_indices.includes(index) ) {
                 fetched_indices.push(index)
-                random_items.push(await this._items.at_index(index))
+                const item = await this._items.at_index(index)
+                if ( typeof item !== 'undefined' )
+                    random_items.push(item)
             }
         }
 

lib/src/collection/Iterable.ts

@@ -8,7 +8,7 @@ export class StopIteration extends Error {}
 export abstract class Iterable<T> {
     protected index = 0
 
-    abstract async at_index(i: number): Promise<T>
+    abstract async at_index(i: number): Promise<T | undefined>
     abstract async from_range(start: number, end: number): Promise<Collection<T>>
     abstract async count(): Promise<number>
     abstract clone(): Iterable<T>