If `count=TRUE`

and `by.tree=FALSE`

, a integer vector containing
frequencies that variables are used in the forest. If

`by.tree=TRUE`

, a matrix is returned, breaking down the counts by
tree (each column corresponding to one tree and each row to a variable).

If `count=FALSE`

and `by.tree=TRUE`

, a list of integer
indices is returned giving the variables used in the trees, else if

`by.tree=FALSE`

, a vector of integer indices giving the
variables used in the entire forest.