if a surface is touched by the fillet, but is not part of the edge
_filletEdge fails.
the red fillet is not found by _filletEdge it has to be done manually by _filletSrf
it would be possible to detect:
if the rolling ball touches additional edges - they might have to be analysed as well.
latest if filleting / trimming the edge fails, it should be possible to analyse the offsets of the surfaces.
all corresponding surface offsets that intersect are affected by the fillet