If no public methods provided and cloneable is FALSE, roxygen2 will failed

[Yunuuuu]

#' R6 Class Representing a Person
#'
#' @description
#' A person has a name and a hair color.
#'
#' @details
#' A person can also greet you.
Person <- R6::R6Class("Person",
    active = list(
        #' @description
        #' Change hair color.
        #' @param val New hair color.
        #' @examples
        #' P <- Person("Ann", "black")
        #' P$hair
        #' P$set_hair("red")
        #' P$hair
        set_hair = function(val) {
            val
        },

        #' @description
        #' Say hi.
        greet = function() {
            cat("Hello")
        }
    ),
    cloneable = FALSE
)