The svn_delta_editor_t interface has served Subversion well from its earliest
days, offering a unified mechanism for describing how to transform a tree of
files and directories and properties into another tree of the same. We use it
commit changes from client to server; we use it to update/switch/merge/diff/etc.
changes from server to client. But it comes with a set of driver constraints
that we now feel aren't able to meet the needs of a more mature Subversion. So,
in short, this critical piece of Subversion's design in need of an overhaul.