diff --git a/prototype/Doubt/Category.swift b/prototype/Doubt/Category.swift
index 1345ff3c0..ae16e3237 100644
--- a/prototype/Doubt/Category.swift
+++ b/prototype/Doubt/Category.swift
@@ -56,4 +56,6 @@ public func < (left: Category, right: Category) -> Bool {
 // MARK: - Categorizable
 
 /// A type whose values belong to a set of categories.
-public protocol Categorizable {}
+public protocol Categorizable {
+	var categories: Set<Category> { get }
+}