This function extracts a designated subset of a point pattern
on a linear network. The function [.lpp is a method for [ for the
class "lpp". It extracts a designated subset of a point pattern.
The argument i should be a subset index in the usual Rsense:
either a numeric vector
of positive indices (identifying the points to be retained),
a numeric vector of negative indices (identifying the points
to be deleted) or a logical vector of length equal to the number of
points in the point pattern x. In the latter case,
the points (x$x[i], x$y[i]) for which
subset[i]=TRUE will be retained, and the others
will be deleted.
The argument j, if present, should be a spatial window.
The pattern inside the region will be retained.
Line segments that cross the boundary of the window
are deleted in the current implementation.
Use the function unmark to remove marks from a
marked point pattern.