Snapshots of individual files

[BP25 <bp25@posteo.net>]

Hello,
Can any of you guys help me understand why it hasn't been made possible 
to snapshot individual files? Because technically it's trivial to 
implement therefore I suspect there must be some abstract reason... The 
only thing I can think of is the case where some file which was 
snapshotted is then deleted hence there is no way to 'select such file' 
and ask btrfs for the snapshotted versions... but even in this case I 
see no problem: either the convention is that when you delete a file 
then all snapshots of such individual file are also deleted, or better 
there is a command that shows all files who have been deleted but have 
have been snapshotted in the past.
Any ideas?
Please CC or BCC me cause I'm not subscribed.