Brep.Trim orientation issue

Hi there,

I'm writing a vb.net component, which attempts to trim a freeform surface (brep1) using an open-ended cylinder (brep2). So far the code is simply:

Dim bb() As Brep

bb = brep1.Trim(brep2, 0.01)

A = bb

The problem is that it doesn't matter which way I flip the input surface normal directions, the trim always returns the larger surface with a circular hole. Actually what I want is just the circular hole. The documentation implies that the orientation of the cylinder should affect which piece is returned.

Any suggestions?

Steven