def draw_medium(r, R, L, n=<NUM_LIT:1>, ax=None):
if ax is None:<EOL><INDENT>ax = plt.gca()<EOL><DEDENT>for ru in _unwrap_to_layer(r, L, n):<EOL><INDENT>c = plt.Circle(ru, radius=R, alpha=<NUM_LIT>)<EOL>ax.add_artist(c)<EOL><DEDENT>
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