From 8d9d27d64d9e39ea76fd878e928e553944735e45 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Fri, 1 Jul 2016 08:58:32 +0200
Subject: Differentiate to more than one variable

---
 rules.txt | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/rules.txt b/rules.txt
index aa1f44d..9a005ee 100644
--- a/rules.txt
+++ b/rules.txt
@@ -64,7 +64,12 @@ Negative {a} | Product [Number (-1),{a}]
 
 ## differentiation
 :d:[{{n:Number}},{x}] | Number 0
+
+-- these two differentiation rules should be tried in THIS order.
+-- Better: guard for d(x,y) that x!=y...
 :d:[{{x:Variable}},{{x}}] | Number 1
+:d:[{{x:Variable}},{{y}}] | Number 0
+
 :d:[Sum [{{a}},{b}],{{x}}] | Sum [:d:[{a},{x}],:d:[{b},{x}]]
 :d:[Product [{{n:Number}},{a}],{x}] | Product [{n},:d:[{a},{x}]]
 :d:[Product [{{a}},{b}],{x}] | Sum [Product [:d:[{a},{x}],{b}],Product [{a},:d:[{b},{x}]]]
-- 
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