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.