A method in the Seq class can be applied to any kind of IEnumerable.  Consider the following code:

let sum l = l |> Seq.reduce (+)

let testArray = [|1..10|]
let testList = [1..10]
let testSeq = seq { 1..10 }

let aSum = sum testArray //55
let lSum = sum testList //55
let sSum = sum testSeq //55