Extension methods for specific generic types

I'm attempting to create various extension method for a generic type bound to specific generic type parameters in F#, but the language does not seem to be allowing me: What I want to do is something like the following: ``` type IEnumerable<int> with member this.foo = this.ToString() ``` Yet it gives me the compiler error (underlining the `int` keyword): Unexpected identifier in type name. Expected infix operator, quote symbol or other token. The following does work, though it does not specifically bind the generic type parameter to `int`, as I want: ``` type IEnumerable<'a> with member this.foo = this.ToString() ``` Is there any way to accomplish this aim in F# - am I perhaps just using the wrong syntax? If not, I would appreciate if someone could suggest a workaround, perhaps using type constraints somewhere.