I just finished reading this paper, and have a question: CAUSAL SITES AS QUANTUM GEOMETRY J. DANIEL CHRISTENSEN AND LOUIS CRANE Abstract. http://arxiv.org/PS_cache/gr-qc/pdf/0410/0410104.pdf We propose a structure called a causal site to use as a setting for quantum geometry, replacing the underlying point set. The structure has an interesting categori-cal form, and a natural “tangent 2-bundle,” analogous to the tangent bundle of a smooth manifold. Examples with reasonable finiteness conditions have an intrinsic geometry, which can approximate classical solutions to general relativity. We propose an approach to quantization of causal sites as well. http://arxiv.org/PS_cache/gr-qc/pdf/0410/0410104.pdf Can anyone define a "tangent 2-bundle?" in plain English? Thanks!!