From 53ea4fcf369a2a0ea1ce19daa729768c334df6ab Mon Sep 17 00:00:00 2001
From: Rob Rix <rob.rix@github.com>
Date: Thu, 15 Oct 2015 11:51:01 -0400
Subject: [PATCH] Interpretation of `.ByIndex` algorithms.

---
 prototype/Doubt/Interpreter.swift | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/prototype/Doubt/Interpreter.swift b/prototype/Doubt/Interpreter.swift
index 0dfb31370..77963a350 100644
--- a/prototype/Doubt/Interpreter.swift
+++ b/prototype/Doubt/Interpreter.swift
@@ -46,8 +46,8 @@ public struct Interpreter<Term: TermType> {
 			let patched = Set(a.keys).intersect(b.keys).map { ($0, run(a[$0]!, b[$0]!)) }
 			return recur(f(Dictionary(elements: deleted + inserted + patched)))
 
-		default:
-			return nil
+		case let .Roll(.ByIndex(a, b, f)):
+			return recur(f(SES(a, b, cost: cost, recur: recur)))
 		}
 	}