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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def check_input_files(self, return_found=True, return_missing=True):
 """Check if input files exist. Parameters return_found : list A list with the paths of the files that were found. return_missing : list A list with the paths of the files that were missing. Returns ------- found : list List of the found files, if requested, otherwise `None` missing : list List of the missing files, if requested, otherwise `None` """

        all_input_files = self.files.chain_input_files + self.sub_files.chain_input_files
        return check_files(all_input_files, self._file_stage,
                           return_found, return_missing)