When I code splitFace triangles I get error.

#light

open System

open System.Drawing

open Microsoft.DirectX

open Microsoft.DirectX.Direct3D

let vec(x, y, z) =

let f = Float32.of_float

new Vector3(f x, f y, f z)

let faces =

let r1 i = 1 + (1 + i) % 5

let r2 i = 6 + (1 + i) % 5

[ for i in 0 .. 4 -> 0, r1 i, r1(i + 1) ] @

[ for i in 0 .. 4 -> r1(i + 1), r1 i, r2 i ] @

[ for i in 0 .. 4 -> r1(i + 1), r2 i, r2(i + 1) ] @

[ for i in 0 .. 4 -> r2(i + 1), r2 i, 11 ]

let vertices =

let r, y = sin(Math.PI / 3.0), cos(Math.PI / 3.0)

let f g x = r * g(float x * Math.PI / 5.0)

let aux n y i = vec(f sin (2 * i + n), y, f cos (2 * i + n))

Array.concat

[ [| vec(0.0, 1.0, 0.0) |]

Array.init 5 (aux 0 y)

Array.init 5 (aux 0 (-y))

[| vec(0.0, -1.0, 0.0) |] ]

let triangles =

[ for i, j, k in faces ->

vertices.[i], vertices.[j], vertices.[k] ]

let splitEdge(p, q) =

Vector3.Normalize(Vector3.Cross(p, q))

let splitFace t (a, b, c) =

let d = splitEdge (a, b)

let e = splitEdge (b, c)

let f = splitEdge (c, a)

[ a, d, f; d, b, e; e, c, f; d, e, f; ]

let subdivide triangles =

List.concat (List.map splitFace triangles) // ERROR