let mid x = 

  [for a in x do 

    for b in x do 

      if a <> b then

        yield a + b

  ] 

  |> set 

  |> Array.ofSeq

let x = [1;9;13;17;21;29]

let y = [3;7;11;19;23;27]

​

printfn "%A" (mid x)

printfn "%A" (mid y)

printfn "%A" (mid x = mid y)

​

​