code: def draw_medium(r, R, L, n=1, ax=None):
      
      if ax is None:
          ax = plt.gca()
      for ru in _unwrap_to_layer(r, L, n):
          c = plt.Circle(ru, radius=R, alpha=0.2)
          ax.add_artist(c)
docs: Draw circles representing circles in a two-dimensional periodic system.
      Circles may be tiled up to a number of periods.
  
      Parameters
      ----------
      r: float array, shape (:, 2).
          Set of points.
      R: float
          Circle radius.
      L: float array, shape (2,)
          System lengths.
      n: integer.
          Period to unwrap up to.
      ax: matplotlib axes instance or None
          Axes to draw circles onto. If `None`, use default axes.
  
      Returns
      -------
      None