def __add__(self, another: Matrix) -> Matrix:

        # Validation
        assert isinstance(another, Matrix)
        assert self.row == another.row and self.column == another.column

        # Add
        result = Matrix(self.row, self.column)
        for r in range(self.row):
            for c in range(self.column):
                result[r, c] = self[r, c] + another[r, c]
        return result