functor KRUSKAL(
structure G: sig
     type Edge
     type Vertex
     type Graph
     val ends : Edge -> Vertex * Vertex (* graph *)
     val edges: Graph -> Edge list
    end
structure PQ: QueueSig  (* a priority queue of edges *)
structure  F: SetSig    (* a set of edges *)
structure P: sig
     type Part
     eqtype Item
     val empty: Part
     val union: Part -> (Item*Item) -> Part
     val find : Part -> Item -> Item
    end
    sharing type F.Item = PQ.Item = G.Edge
        and type P.Item = G.Vertex
) = struct
   type Graph  = G.Graph
    and Forest = F.Set

   fun span (g: Graph) : Forest =
   let fun grow edgeQ forest p =
           if PQ.isEmpty edgeQ then forest
           else
           let val (e,q) = PQ.deq edgeQ 
               val (u,v) = G.ends e
           in if P.find p u = P.find p v  
                 then grow q forest p
              else grow q 
                       (F.insert(e, forest)) 
                       (P.union p (u,v))
           end
    in
       grow (PQ.menq (G.edges g, PQ.empty)) F.empty P.empty
    end 
end;