R help - Oct 2017 - fraction of null deviance explained by each node/variable in regression trees

I have used packages rpart, mvpart and tree for classification and
regression trees. I want to calculate fraction of null deviance explained
by each node and variable in the tree. For instance, at the first split,
this would be (1 - (sum of residual deviance in each of the two
leaves)/deviance at the root). In the subsequent splits, this formula is
slightly different.

There probably is a function to get this done within R. Any help is
appreciated.

Thank you,
Kumar Mainali

-- 
Postdoctoral Associate
Department of Biology
University of Maryland, College Park
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