type FooBuilder() =
    member _.Yield(x) = x
    member _.Zero() = ()
let foo = FooBuilder()
let x = foo { 3 }
val x : unit = ()

type FooBuilder() =
    member _.Yield(x: int) = x
    member _.Combine(u: unit, x: int) = x
    member _.Zero() = ()
let foo = FooBuilder()
let x = foo { 3 }

type BBBbuilder() =
    member _.Zero() = []
    member _.Yield x = [x]
    member _.Combine (x,y) = x @ y
    member _.Delay f = f()
    
let b = BBBbuilder()


let c = 
    b {
        1
        2
        3
    }