Why does Fsharp.Core use GetEnumerator instead

I was looking at implementing F # Seq methods - https://github.com/fsharp/fsharp/blob/master/src/fsharp/FSharp.Core/seq.fs .

Here's an example of a scan implementation:

[<CompiledName("Scan")>]
let scan<'T,'State> f (z:'State) (source : seq<'T>) = 
    checkNonNull "source" source
    let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(f)
    seq { let zref = ref z
          yield !zref
          use ie = source.GetEnumerator() 
          while ie.MoveNext() do
              zref := f.Invoke(!zref, ie.Current)
              yield !zref }

      

        
        
        
      

    

It gets an enumerator and uses a while loop to iterate (it should call MoveNext).

Can't this be changed?

[<CompiledName("Scan")>]
let scan<'T,'State> f (z:'State) (source : seq<'T>) = 
    checkNonNull "source" source
    let f = OptimizedClosures.FSharpFunc<_,_,_>.Adapt(f)
    seq { let zref = ref z
          yield !zref
          for i in source do
              zref := f.Invoke(!zref, i)
              yield !zref }

      

        
        
        
      

    

What does the for loop use.

Is it just that someone with an odd coding style wrote this code and no one changed it?