sync

sync operation:

we have 2 sets, and want to keep them in synchronization.

1. Find 2 bijection.
2. then a -> f(g(a)) is an oriented graph.

3 divide the graph into: oriented (acyclic) paths and cycles.

4 `move' along the cycles & paths.

--- moves can be sys-rename db:update ...

--- what bijections are available:

->: foto -> file by hash/md5 <-: file -> foto by position graph foto -> foto

->: backup file -> file/foto by md5 <-: -> position