allow for pm to be slowly filled when computing products

src/GroupRings.jl

@@ -351,6 +351,10 @@ function mul!{T}(result::GroupRingElem{T}, X::GroupRingElem, Y::GroupRingElem)
       if Y.coeffs[j] != z
          for i in eachindex(X.coeffs)
             if X.coeffs[i] != z
+               if parent(X).pm[i,j] == 0
+                  g = parent(X).basis[i]*parent(Y).basis[j]
+                  parent(X).pm[i,j] = parent(X).basis_dict[g]
+               end
                result.coeffs[parent(X).pm[i,j]] += X[i]*Y[j]
             end
          end