tree \code{x} where the tree \code{y} is binded (\code{"root"} is a
short-cut for the root).}
\item{position}{a numeric value giving the position from the tip or
tree \code{x} where the tree \code{y} is binded (\code{"root"} is a
short-cut for the root).}
\item{position}{a numeric value giving the position from the tip or