clickjoin(X, ..., add = TRUE, m = NULL, join = TRUE)"ppp".segments to control
    the plotting of the new edges.X should be added to the
    existing plot (add=TRUE) or a new plot should be created
    (add=FALSE).i and
    j if m[i,j] = TRUE.TRUE, then each user click will join a pair of
    vertices. If FALSE, then each user click will delete an
    existing edge. This is only relevant if m is supplied.m with value m[i,j] = TRUE for every
  pair of vertices X[i] and X[j] that should be joined by
  an edge.X,
  then repeatedly prompts the user to click on a pair of points in X.
  Each selected pair of points will be joined
  by an edge. The function returns a
  logical matrix which has entries equal to TRUE for each
  pair of vertices joined by an edge.  The selection of points is performed
  using identify.ppp which typically expects the user to
  click the left mouse button.  This point-and-click interaction
  continues until the user terminates it, 
  by pressing the middle mouse button,
  or pressing the right mouse button and selecting stop.
  The return value can be used in linnet
  to create a linear network.
linnet,
  clickppp