Honestly it's MutableCollectionType

Robert Thompson · Robert Thompson · commit d88f10e94866 · 2016-02-18T18:01:22.000-05:00
diff --git a/Treap/TreapCollectionType.swift b/Treap/TreapCollectionType.swift
@@ -8,7 +8,7 @@
 
 import Foundation
 
-extension Treap: CollectionType {
+extension Treap: MutableCollectionType {
     public typealias Index = TreapIndex<Key>
 
     public subscript(index: TreapIndex<Key>) -> Element {
