anchor.diff:    ### NixOS - usage:
     ==================
   
  -    services.posgresql.extraPlugins = [ pkgs.postgis.v_1_5_1 ];
  -
  -    services.postgresql.extraPlugins = [ (pkgs.postgis.override { postgresql = pkgs.postgresql94; }).v_2_1_4 ];  # BM - 2015/01/01
  -
  -    or if you want to install 1.5.x and 1.3.x at the same time (which works
  -    because the .sql and .so files have different names):
  -
  -    services.postgis.extraPlugins = [ (pkgs.buildEnv {
  -          name = "postgis-plugins";
  -          ignoreCollisions = 1; # scripts will collide - but there are aliases with version suffixes
  -          paths = [ pkgs.postgis.v_1_3_5 pkgs.postgis.v_1_5_1 ];
  -        })];
  -
  -    By now it is only supported installing one of the 1.3.x verions because
  -    their shared libraries don't differ in naming.
  -
  +    services.postgresql.extraPlugins = [ (pkgs.postgis.override { postgresql = pkgs.postgresql94; }).v_2_1_4 ];
   
   
     ### important Postgis implementation details:
  
positive.code: 
        CREATE FUNCTION [...]
                AS '[..]liblwgeom', 'lwhistogram2d_in' LANGUAGE 'C' IMMUTABLE STRICT; -- WITH (isstrict);
      
     where liblwgeom is the shared library.
     Postgis < 1.5 used absolute paths, in NixOS $libdir is always used.
  
     Thus if you want to use postgresql dumps which were created by non NixOS
     systems you have to adopt the library path.
  
  
  
     ### TODO:
  
negative.code: 
  in rec {
  
    v_2_1_4 = pgDerivationBaseNewer.merge ( fix : {
      version = "2.1.4";
      sha256 = "1z00n5654r7l38ydkn2grbwl5gg0mravjwxfdipp7j18hjiw4wyd";
  