#' Does the tree contain a given key?
#'
#' @param tree A \code{bst} object
#' @param key The key to search for
#'
#' @details returns TRUE or FALSE
#' @export
contains <- function(tree, key) UseMethod("contains")
#' @export
contains.bst <- function(tree, key) !is.null(retrieve(tree, key))
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