snip.tree has two related functions. If nodes is
supplied, it removes those nodes and all their descendants from the
tree. If nodes is not supplied, the user is invited to select
nodes interactively; this makes sense only if the tree has already been
plotted. A node is selected by clicking with the left mouse button;
its number and
the deviance of the current tree and that which would remain if that
node were removed are printed. Selecting the same node again causes it
to be removed (and the lines of its sub-tree erased).
Clicking any other button terminates the selection process.
snip.tree(tree, nodes, xy.save = FALSE,
digits = getOption("digits") - 3)"tree".x and y coordinates selected
interactively are saved in the object .xy in the global
environment.tree, prune.tree.