While as OSCON, I was talking with some people who want to embed SVN in their program. In order to dynamically allocate some of the structures, they need a context pointer to hold their private state (e.g. the target of a thunking vtable layer). To help these people out, we simply add a little context field to some of our structures. Normally, this will be NULL (especially if the structure is statically allocated constant data).
* svn_delta.h (svn_delta_editor_t): add a void* context field.