code: def solid(self, x, y):
          
          if not(0 <= x < self.xsize) or not(0 <= y < self.ysize):
              return False
          if self.data[x, y] == 0:
              return False
          return True
docs: Determine whether the pixel x,y is nonzero
  
          Parameters
          ----------
          x, y : int
              The pixel of interest.
  
          Returns
          -------
          solid : bool
              True if the pixel is not zero.