An object of class (boostmtree, grow) or (boostmtree,
predict).
...
Further arguments passed to or from other methods.
References
Pande A., Li L., Rajeswaran J., Ehrlinger J., Kogalur U.B.,
Blackstone E.H., Ishwaran H. (2017). Boosted multivariate trees for
longitudinal data, Machine Learning, 106(2): 277--305.