diff --git a/prototype/Doubt/Algorithm.swift b/prototype/Doubt/Algorithm.swift
index 47c8a2358..c9d97b60c 100644
--- a/prototype/Doubt/Algorithm.swift
+++ b/prototype/Doubt/Algorithm.swift
@@ -113,6 +113,8 @@ extension Algorithm where B: FreeConvertible, B.RollType == Term.LeafType, B.Pur
 	/// Evaluate the algorithm.
 	///
 	/// `equals` compares two terms for equality. It should be strictly syntactic equality, ignoring any annotations/metadata on the terms.
+	///
+	/// `categorize` provides sets of categories for terms. If the categories of two terms are distinct and non-intersecting, then the terms should not be compared in e.g. SES. This is provided as a parameter so that `Term` need not be `Categorizable` in order to use it.
 	public func evaluate<C>(equals: (Term, Term) -> Bool, categorize: Term -> Set<C>) -> B {
 		return evaluate(equals, recur: {
 			let c0 = categorize($0)