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: 
  in rec {
  
    # these builders just add some custom informaton to the receipe above
  
    v_1_3_5 = pgDerivationBase.merge ( fix: {
      version = "1.3.5";
      buildInputs = [ flex ];
      sha256 = "102d5ybn0db1wrb249dga2v8347vysd4f1brc8zb82d7vdd34wyq";
      sql_srcs = ["lwpostgis.sql" "spatial_ref_sys.sql"];
  
      pg_db_postgis_fix_or_load_sql_dump = ./pg_db_postgis_fix_or_load_sql_dump.sh;
      libName = "liblwgeom";
      scriptNames = [ "pg_db_postgis_enable" "pg_db_postgis_fix_or_load_sql_dump"]; # helper scripts
  
      # sql_srcs is defined in postInstall source above
      # if store path changes sql should not break. So replace absolute path to
      # shared library by path relatve to $libdir known by Postgres.
      postInstall = ''
        sed -i "s@AS '$out/lib/liblwgeom@AS '\$libdir/liblwgeom@" $sql_srcs $out/share/lwpostgis_upgrade.sql
      '';
    });
  
    v_1_3_6 = v_1_3_5.merge ({
      version = "1.3.6";
      sha256 = "0i6inyiwc5zgf5a4ssg0y774f8vn45zn5c38ccgnln9r6i54vc6k";
    });
  
    v_1_5_1 = pgDerivationBase.merge ( fix : {
      version = "1.5.1";
      sha256 = "0nymvqqi6pp4nh4dcshzqm76x4sraf119jp7l27c2q1lygm6p6jr";
      sql_srcs = ["postgis.sql" "spatial_ref_sys.sql"];
    });
  
    v_1_5_8 = pgDerivationBaseNewer.merge ( fix : {
      version = "1.5.8";
      sha256 = "016qn2hk3w8rg60rqn91l2nfk9zcsw0pv2s5rb1qhjw15ypgv5j8";
      sql_srcs = ["postgis.sql" "spatial_ref_sys.sql"];
    });
  
    v_2_0_6 = pgDerivationBaseNewer.merge ( fix : {
      version = "2.0.6";
      sha256 = "129lyd1crv1bbjadyisjm3wv2vkj1v1nnxyns9z57300ip7wk3s3";
      sql_srcs = ["postgis.sql" "spatial_ref_sys.sql"];
      builtInputs = [gdal pkgconfig];
  
      preConfigure = ''
        sed -i 's@/usr/bin/file@${file}/bin/file@' configure
        configureFlags="$configureFlags --with-gdalconfig=${gdal}/bin/gdal-config"
      '';
    });
  
    v_2_1_4 = pgDerivationBaseNewer.merge ( fix : {
      version = "2.1.4";
      sha256 = "1z00n5654r7l38ydkn2grbwl5gg0mravjwxfdipp7j18hjiw4wyd";
  
negative.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:
  