collapsing commits with rebase

Dear gits,

I have a series of commits:

    A---B---C---D---E---F

I want to collapse B---C---D into one single commit. git rebase -i B will allow me to do this, but I'm looking for a non-interactive incantation.

Cheers, Geoff Russell

P.S. The context is a program that performs a single high level operation on a repository as a series of commits but then wants to turn it back into a single commit without user intervention so it subsequently looks like a single op in the history.