anchor.diff:        , this, self, newScope, buildEnv
   
         # source specification
  -      , version, sha256, psqlSchema
  +      , version, sha256, psqlSchema,
  +
  +      # for tests
  +      nixosTests, thisAttr
       }:
     let
       atLeast = lib.versionAtLeast version;
  
positive.code:     psqlSchema = "9.5";
      sha256 = "0an2k4m1da96897hyxlff8p4p63wg4dffwsfg57aib7mp4yzsp06";
      this = self.postgresql_9_5;
      inherit self;
    };
  
  
negative.code:     psqlSchema = "10.0"; # should be 10, but changing it is invasive
      sha256 = "0zhzj9skag1pgqas2rnd217vj41ilaalqna17j47gyngpvhbqmjr";
      this = self.postgresql_10;
      thisAttr = "postgresql_10";
      inherit self;
    };
  
  