find a minimum spanning tree in different sets :   
  Here I have two connected undirected graphs G1 = [V ; E1] and G2 =
[V ; E2] on the same set of vertices V . And assume edges in E1 and E2 have different colors. Let w(e) be the weight of edge e ∈ E1 ∪ E2.
   
   I want to find a minimum weight spanning tree (MSF) among those spanning trees which have at least one edge in each set E1 and E2. In this condition, how to find a proper algorithm for this? I got stuck here a whole night. 
