diff --git a/prototype/Doubt/Category.swift b/prototype/Doubt/Category.swift
index d37a64228..2edc6d431 100644
--- a/prototype/Doubt/Category.swift
+++ b/prototype/Doubt/Category.swift
@@ -1,3 +1,4 @@
+/// A syntactic category to which nodes can belong.
 public enum Category: AlgebraicHashable, Comparable, CustomDebugStringConvertible, CustomJSONConvertible {
 	case Tag(String)