<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def blit(self, surface, pos=(0, 0)):
 """ Blits a surface on this surface at pos :param surface: Surface to blit :param pos: Top left point to start blitting :type surface: Surface :type pos: tuple """

        for x in range(surface.width):
            for y in range(surface.height):
                px = x + pos[0]
                py = y + pos[1]
                if 0 < px < self.width and 0 < py < self.height:
                    self.matrix[px][py] = surface.matrix[x][y]