  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.
