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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 diff_lines(self):
 """A diff between the original BUILD file and the resulting BUILD file."""

    start_lines = self._build_file_source_lines[:]
    end_lines = self.build_file_lines()
    diff_generator = unified_diff(start_lines,
                                  end_lines,
                                  fromfile=self.build_file.relpath,
                                  tofile=self.build_file.relpath,
                                  lineterm='')
    return list(diff_generator)