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#light

module LazyListMonad =
    let result v = LazyList.cons v (LazyList.empty())

    let bind (llist:'a LazyList) f = LazyList.map f llist |> LazyList.concat

    let zero<'a> = LazyList.empty(): 'a LazyList

    let append xs ys = LazyList.append xs ys

    type LazyBuilder() =
        member b.Bind (llist, f) = bind llist f
        member b.Return (v) = result v
        member b.Let (v, f) = bind (result v) f
        member b.For (s, f) = bind (LazyList.of_seq s) f
        member b.Combine (listA, listB) = LazyList.append listA listB
        member b.Zero () = LazyList.empty ()
        member b.Delay (f) = f():'a LazyList
    
let LazyL = new LazyListMonad.LazyBuilder()

let list = 
    LazyL
        {  for i in 1 .. 6 do
            for j in 10 .. 20 do 
                if i%2=0 && j%2=0 then
                    yield (i,j)
        }