Sequenced output should contain pointer back to the content from which it originated

The root of the sequenced output should contain a pointer back to the content that was originally sequenced. For example, we might define a 'mode:sequenced' mixin that contains a PATH property named 'mode:sequencedFrom'; when the sequencer runs, it adds this mixin and this property (with a value pointing back to the node that was the input to the sequencing operation) to the output node.

There are a couple of issues to work out.
1) What happens if/when the sequenced output contains multiple root nodes? Technically this is possible, though none of our existing sequencers do this. Perhaps we add it to each of the root nodes??
2) We can't really use a REFERENCE property, as the sequenced node might not be 'mix:referenceable'. If it were a REFERENCE, though, it would properly handle moves.
3) If the sequenced input is a file (as is so often the case), the actual sequenced node would likely be the "jcr:content" node under the "nt:file" node. Should the path point back to this, or to the "nt:file" node?

This would also provide a way for the sequencing operation to replace any existing content (under the specified output path) that was the output of a previous sequencing operation on the same input path.