Add set/array operations to $unwind in new aggregation framework

$unwind operates on a single array field. sometimes tag-like information can be stored in multiple document fields. it may be useful to allow $unwind to operate on multiple array fields whose contents have been combined by some set/array operation. this could help users group by values that are contained across multiple fields.

i.e.

$unwind(x).$unwind(y) would create four documents.

would something like $unwind($union(x,y)) and group on $union(x,y) be possible/desirable?