The Stienen diagram of a point pattern (Stienen, 1982)
is formed by drawing a circle around each point of the pattern,
with diameter equal to the nearest-neighbour distance for that point.
These circles do not overlap. If two points are nearest neighbours
of each other, then the corresponding circles touch.

`stienenSet(X)`

computes the union of these circles and
returns it as a window (object of class `"owin"`

).

`stienen(X)`

generates a plot of the Stienen diagram of
the point pattern `X`

. By default, circles are shaded in grey
if they lie inside the window of `X`

, and are not shaded
otherwise.