From 60c7eac861a34acc1e490eb66a42e059d9fb9955 Mon Sep 17 00:00:00 2001
From: Jeremy Dormitzer <jeremy.dormitzer@gmail.com>
Date: Sun, 14 Jul 2019 23:03:29 -0400
Subject: [PATCH] Fix negation

(to the broken version of "negation" given in SICP)
---
 src/sicp_logic/evaluator.clj | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/sicp_logic/evaluator.clj b/src/sicp_logic/evaluator.clj
index b081b10..06a815a 100644
--- a/src/sicp_logic/evaluator.clj
+++ b/src/sicp_logic/evaluator.clj
@@ -19,9 +19,11 @@
             (disjoin db (rest disjuncts) input-frames))))
 
 (defn negate [db operands input-frames]
-  (filter
+  (mapcat
    (fn [frame]
-     (empty? (qeval db (first operands) [frame])))
+     (if (empty? (qeval db (first operands) [frame]))
+       [frame]
+       []))
    input-frames))
 
 (defn execute [exp]