SPARQL interaction of OPTIONAL, FILTER, and BIND

This is completely unrelated to any recent questions about the rule
engine and builtins. :) I've got this data:

@prefix : <http://example.org/> .
:a :atIndex 0 .
:b :atIndex 1 .
:c :atIndex 2 .
:d :atIndex 3 .

Here's a query that finds an ?element at an ?index, binds ?index2 to
?index - 1, and optionally selects an ?element2 that is at ?index2.

prefix : <http://example.org/>
select ?element ?index ?element2 ?index2 where {
?element :atIndex ?index .
BIND( ?index - 1 as ?index2 )
OPTIONAL {
?element2 :atIndex ?index2 .

order by ?index

The results are as expected:

$ arq --data data.n3 --query query.sparql

| element | index | element2 | index2 |
=======================================
| :a | 0 | | -1 |
| :b | 1 | :a | 0 |
| :c | 2 | :b | 1 |
| :d | 3 | :c | 2 |

Moving the BIND into the OPTIONAL, i.e.,

prefix : <http://example.org/>
select ?element ?index ?element2 ?index2 where {
?element :atIndex ?index .
OPTIONAL {
BIND( ?index - 1 as ?index2 )
?element2 :atIndex ?index2 .

order by ?index

produces lots more results; it seems that there's no constraint on
the ?index within the OPTIONAL, so ?index2 can range over everything.

$ arq --data data.n3 --query query.sparql

| element | index | element2 | index2 |
=======================================
| :a | 0 | :a | 0 |
| :a | 0 | :b | 1 |
| :a | 0 | :c | 2 |
| :a | 0 | :d | 3 |
| :b | 1 | :a | 0 |
| :b | 1 | :b | 1 |
| :b | 1 | :c | 2 |
| :b | 1 | :d | 3 |
| :c | 2 | :a | 0 |
| :c | 2 | :b | 1 |
| :c | 2 | :c | 2 |
| :c | 2 | :d | 3 |
| :d | 3 | :a | 0 |
| :d | 3 | :b | 1 |
| :d | 3 | :c | 2 |
| :d | 3 | :d | 3 |

Now, using a FILTER in the OPTIONAL:

prefix : <http://example.org/>

select ?element ?index ?element2 ?index2 where {
?element :atIndex ?index .
OPTIONAL {
FILTER( ?index - 1 = ?index2 )
?element2 :atIndex ?index2 .

order by ?index

produces results that actually show that ?index2 is constrained, and
are almost the same the as the first query (except that the case where
?index2 is -1 doesn't occur):

$ arq --data data.n3 --query query.sparql

| element | index | element2 | index2 |
=======================================
| :a | 0 | | |
| :b | 1 | :a | 0 |
| :c | 2 | :b | 1 |
| :d | 3 | :c | 2 |

Is this expected? Why can't the BIND inside the optional reference
the outer binding of ?index? In the spec, I do read about the
difference of scope between, e.g., FILTER NOT EXISTS { ?n ... } and
MINUS { ?n ... }, where in the former, ?n can refer to an outer
binding, but for the latter, it cannot. I think that 18.2.1 Variable
Scope [1] is probably relevant here, but I'm not sure how to get an
answer from it.

Thanks in advance!
//JT

[1] http://www.w3.org/TR/2013/REC-sparql11-query-20130321/#variableScope