Make the nice labels used by print and summary.
# S3 method for rpart
labels(object, digits = 4, minlength = 1L, pretty,
collapse = TRUE, ...)
obvious
0 = don't abbrev factors; 1 = use single letters; 2+= the same arg as the "abbreviate" function
an oddly named argument: FALSE = return a matrix with two columns, containing the labels of the left and right descendants of each node; TRUE = return a vector of 1 column, with the label of the parent
= other args for abbreviate()
for historical compatability: 0 -> minlength = 0; NULL -> minlength = 1; TRUE -> minlength = 4