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

def draw_medium(r, R, L, n=1, ax=None):
    """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
    """
    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)