Abstract Separation Systems
  Abstract separation systems provide a simple general framework in which both tree-shape and high cohesion of many combinatorial structures can be expressed, and their duality proved. Applications range from tangle-type duality and tree structure theorems in graphs, matroids or CW-complexes to, potentially, image segmentation and cluster analysis.   This paper is intended as a concise common reference for the basic definitions and facts about abstract separation systems in these and any future papers using this framework. 
Reinhard Diestel
math.CO