code: def get_stage(self, matrix_name, stage_name):
          
          found_stage = None
          if matrix_name in self.data:
              result = Select(self.data[matrix_name]).where(
                  lambda entry: entry.stage == stage_name).build()
              found_stage = result[0] if len(result) > 0 else None
          return found_stage
docs: Get Stage of a concrete matrix.
  
          Attributes:
              matrix_name (str): name of the matrix
              stage_name (str): name of the stage.
  
          Returns:
              CollectorStage: when stage has been found or None.