From cb15964bfd7c4513bb15e411de70cf611488fa81 Mon Sep 17 00:00:00 2001
From: kalmar <kalmar@amu.edu.pl>
Date: Tue, 20 Dec 2016 17:57:42 +0100
Subject: [PATCH] Warn in multiplication of elts of K[G], when discarding a
 product

---
 GroupAlgebras.jl | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/GroupAlgebras.jl b/GroupAlgebras.jl
index cff32e1..e1740f3 100644
--- a/GroupAlgebras.jl
+++ b/GroupAlgebras.jl
@@ -71,9 +71,15 @@ function group_star_multiplication{T<:Number}(X::GroupAlgebraElement{T},
 
     result = zeros(X.coordinates)
     for (i,x) in enumerate(X.coordinates), (j,y) in enumerate(Y.coordinates)
-        index = X.product_matrix[i,j]
-        if index != 0
-            result[index]+= x*y
+        if x*y == 0
+            nothing
+        else
+            index = X.product_matrix[i,j]
+            if index == 0
+                throw(DomainError("The product don't seem to belong to the span of basis!"))
+            else
+                result[index]+= x*y
+            end
         end
     end
     return GroupAlgebraElement(result, X.product_matrix)