sequence expression headache

gistfile1.fs

open System

let fromIEnumerator (xs: System.Collections.Generic.IEnumerator<'x>): seq<'x> =
  seq { 
    while xs.MoveNext() do
      yield xs.Current
  }

let fromIEnumerable (xs: System.Collections.Generic.IEnumerable<'x>): seq<'x> = fromIEnumerator (xs.GetEnumerator())

let a = new ResizeArray<int>()
a.Add(5)
a.Add(6)

let enum = fromIEnumerable a


// ****************   here's what happens if you call Seq.head on enum over and over: *****************************

> Seq.head enum;;
val it : int = 5
> Seq.head enum;;
val it : int = 6
> Seq.head enum;;
System.ArgumentException: The input sequence was empty.
Parameter name: source
  at Microsoft.FSharp.Collections.SeqModule.Head[Int32] (IEnumerable`1 source) [0x00000] in <filename unknown>:0 
  at <StartupCode$FSI_0014>.$FSI_0014.main@ () [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
Stopped due to error