[Evolution] looking for definition of vfolder scheme language

[HvR <hvrietsc myrealbox com>]

the terse documentation of vfolders for evolution states there is a way
to enter an expression (if your are a programmer) using the scheme
language. Can someone enlighten me on how this is done? i  like vfolders
but i would like a lot more control even the ability to execute external
programs as part of the vfolder filter. 

anyone?