Warn in multiplication of elts of K[G], when discarding a product

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)