<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_stage(self, matrix_name, stage_name):
 """ 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. """

        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