[R] tree model with at most one split point per variable

[Gabor Grothendieck]

I would like to create a tree model with at most one split point per variable
using tree, rpart or other routine.  Its OK if a variable enters at more
than one node but if it does then all splits for that variable should be
at the same point.  The idea is that I want to be able to summarize the
data as binary factors with the chosen split points.  I don't want to
have three level or more factors.

For example, the following shows that the first split is with Petal.Length
splitting at 2.45; however, there are other splits of Petal.Length at
4.95.  I want to disallow that.

R> data(iris)
R> tree(Species ~., data = iris)
node), split, n, deviance, yval, (yprob)
      * denotes terminal node

 1) root 150 329.600 setosa ( 0.33333 0.33333 0.33333 )  
   2) Petal.Length < 2.45 50   0.000 setosa ( 1.00000 0.00000 0.00000 ) *
   3) Petal.Length > 2.45 100 138.600 versicolor ( 0.00000 0.50000 0.50000 )  
     6) Petal.Width < 1.75 54  33.320 versicolor ( 0.00000 0.90741 0.09259 )  
      12) Petal.Length < 4.95 48   9.721 versicolor ( 0.00000 0.97917 
0.02083 )  
        24) Sepal.Length < 5.15 5   5.004 versicolor ( 0.00000 0.80000 
0.20000 ) *
        25) Sepal.Length > 5.15 43   0.000 versicolor ( 0.00000 1.00000 
0.00000 ) *
      13) Petal.Length > 4.95 6   7.638 virginica ( 0.00000 0.33333 0.66667 ) *
     7) Petal.Width > 1.75 46   9.635 virginica ( 0.00000 0.02174 0.97826 )  
      14) Petal.Length < 4.95 6   5.407 virginica ( 0.00000 0.16667 0.83333 ) *
      15) Petal.Length > 4.95 40   0.000 virginica ( 0.00000 0.00000 1.00000 ) 
*