Reference class containing class "list" in R

Question

I basically would like to create a new type of list with Reference Class. Instead of using S3 object, I also want to avoid static methods and define private functions within the reference class, such as:

weirdList<-setRefClass("weirdList"
                   ,contains="list"
                   ,methods=list(
                   ,initialize=function(...){
                     return(list(...))
                   },
                   sumup=function(){
                     sum(list()) # don't know how to write this
                   }))

new_list<-weirdList(a=1,b=2,c=3)

new_list
# expecting output:
# $a
# [1] 1
#     
# $b
# [1] 2
# 
# $c
# [1] 3

new_list$sumup()
# I want to output 6

Obviously the above doesn't work. I find it difficult to "clone" or modify a primitive methods. Does anyone know how to do it? Thanks in advance.