optional character string specifying the name of the
distribution to use or list with a component name specifying the
distribution and any additional parameters needed. Set automatically
according to the class type of the response variable.
n.trees
total number of trees to fit.
interaction.depth
maximum depth of variable interactions.
n.minobsinnode
minimum number of observations in the trees terminal
nodes.
shrinkage
shrinkage parameter applied to each tree in the expansion.
bag.fraction
fraction of the training set observations randomly
selected to propose the next tree in the expansion.