Different indentation level changes behaviour of the code

``` fsharp
let foo t = "(" + t + ")"

let tttt =
let text =
match Some "" with
| Some t ->
match t with
| "" ->
"a"
| "b" ->
"b"
|> foo
| _ -> "fooo"
text

let tttt' =
let text =
match Some "" with
| Some t ->
match t with
| "" ->
"a"
| "b" ->
"b"
|> foo
| _ -> "fooo"
text
```

In `foo` I use two spaces for indentation, in `foo'` four spaces are used.

The resulting expressions are different while I'd had expected those to be the same.

I tried to find something which would explain that in the language specification (which I haven't thoroughly read) but failed, and the behaviour seems to fail me at "principle of least surprise" so submitting this as an issue to get clarification.

Thanks.