Seq模块的静态扩展方法

this post ,F#支持对象实例和静态类上的扩展方法。例如:

module CollectionExtensions = 
    type System.Linq.Enumerable with   
        static member RangeChar(first:char, last:char) = {first .. last}

open ExtensionFSharp.CollectionExtensions 

System.Linq.Enumerable. 静态法 RangeChar 出现在我的智能感知窗口中。

我想添加一个静态方法, for_alli

module SeqExtensions =
    type Microsoft.FSharp.Collections.Seq with   (* error on this line *)
        static member for_alli f l =
            l
            |> Seq.mapi (fun i x -> i, x)
            |> Seq.for_all (fun (i, x) -> f i x)

虽然两段代码的结构相同, SeqExtensions 不编译。F#突出显示单词 Seq

如何在Seq模块上创建静态扩展方法?