class DetailViewMapping(schema.Annotation):
    """
    A mapping between a Kind and the detail view (actually, its rows of 
    blocks) that should appear when an Item inheriting from that Kind 
    is displayed.
    """
    schema.kindInfo(annotates=schema.Kind)
    detailRows = schema.Sequence(inverse=DetailRow.detailViewMapping)

class DetailRow(schema.Item):
    """ 
    One horizontal slice of the detail view 
    """
    detailViewMapping = schema.One() # DetailViewMapping.detailRows
    position = schema.One(schema.Float)
    blocks = schema.Sequence(inverse=DetailBlock.detailRow)
    [...]

class DetailBlock(schema.Annotation):
    """ 
    A block in the detail view - we annotate it with the row it belongs to
    """
    schema.kindInfo(annotates=Block.Block)
    detailRow = schema.One() # detailRow.blocks

    schema.addClouds(
        copying = schema.Cloud(
            byRef = [detailRow]
        )
    )

Building a detail view for an item
- collect the DetailViewMappings that exist for any of the item's kinds
- make a decorated list of the DetailRows belonging to those mappings; sort it by position
- copy a stub block that'll be the root of the new detail view
- copy each of the blocks in the DetailRows list onto the stub copy's childBlocks list.

As specified above, DetailBlock.detailRow is copied byRef, which means that each time I
copy a row's blocks, extra copies of blocks get added to the detailRow.blocks list :-(

If I copy the detailRow too, then I'll get an extra detailRow copy per block, bad (and
the DetailViewMapping.detailRows list will get extra blocks.