  We prove global convergence of a bundle trust region algorithm for non-smooth
non-convex optimization, where cutting planes are generated by oracles
respecting four basic rules. The benefit is that convergence theory applies to
a large variety of methods encountered in practice. This includes in particular
the method of downshifted tangents, for which previously no convergence result
in the trust region framework was known. We also show that certain splitting
techniques can be seen as special cases of bundle trust region techniques.
