def point(self, x, y, char):
        """Create a point on ASCII canvas.

        Args:
            x (int): x coordinate. Should be >= 0 and < number of columns in
                the canvas.
            y (int): y coordinate. Should be >= 0 an < number of lines in the
                canvas.
            char (str): character to place in the specified point on the
                canvas.
        """
        assert len(char) == 1
        assert x >= 0
        assert x < self.cols
        assert y >= 0
        assert y < self.lines

        self.canvas[y][x] = char