Categorizing Different Approaches to the Cosmological Constant Problem
  We have found that proposals addressing the old cosmological constant problem come in various categories. The aim of this paper is to identify as many different, credible mechanisms as possible and to provide them with a code for future reference. We find that they all can be classified into five different schemes of which we indicate the advantages and drawbacks.   Besides, we add a new approach based on a symmetry principle mapping real to imaginary spacetime. 
Stefan Nobbenhuis