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 = "10.0"; # should be 10, but changing it is invasive
      sha256 = "0zhzj9skag1pgqas2rnd217vj41ilaalqna17j47gyngpvhbqmjr";
      this = self.postgresql_10;
      inherit self;
    };
  
  
negative.code:     psqlSchema = "12";
      sha256 = "15gzg778da23sbfmy7sqg443f9ny480301lm7i3vay4m3ls2a3dx";
      this = self.postgresql_12;
      thisAttr = "postgresql_12";
      inherit self;
    };
  
  