anchor.diff:  };
   
   void IntroduceNewShaderVar(String canonical, ShaderVarDesc desc) {
  -  assert(!_VarsDb.containsKey(canonical));
  +  assert(!_VarsDb.containsKey(canonical), "duplicate shader variable ${canonical}");
     _VarsDb[canonical] = desc;
   }
   
  
positive.code:     transformVars.sort();
    }
  
    void SetBodyWithMain(List<String> body, {List<String> prolog = null}) {
      assert(shader == null);
      shader = _CreateShader(true, body, prolog);
    }
  
    void SetBody(List<String> body, {List<String> prolog = null}) {
      assert(shader == null);
      shader = _CreateShader(false, body, prolog);
    }
  
negative.code:       out.add("}");
      }
  
      // print (out.join("\n"));
      return out.join("\n");
    }
  }
  