setRefClass in package

Hello,

I was wondering how to call a function outside a setRefClass but inside
the package without export it. Let me explain by means of an example.

- in the file test-package/R/test.R

##' some description
##'
##' some details
##' @title test
##' @return sideeffect
##' @author Jeremie Juste
##' @export test
##' @import data.table
test <- setRefClass("test",
             list(dt="data.table"))


test$methods(
   
   initialize = function(x){
     dt <<- remove_if_all_na(x[,abc:=1])
     }
)


##' remove rows for which all values are NA
##'
##' @title remove_if_all_NA
##' @param dt
##' @return dt
##' @author Jeremie Juste
remove_if_all_NA <- function(dt) {
   cn <- colnames(dt)
   dt[!dt[NA],on=cn]
}


Here when I build and install the package test-package, if I don't export
remove_if_all_NA

##' remove rows for which all values are NA
##'
##' @title remove_if_all_NA
##' @param dt
##' @return dt
##' @author Jeremie Juste
##' @export
remove_if_all_NA <- function(dt) {
   cn <- colnames(dt)
   dt[!dt[NA],on=cn]
}

The package cannot use it.

library(test-package)
library(data.table)

aa <- data.table(a=1:10,b=letters[1:10])
b <- test(aa)

Error in remove_if_all_na(x[, `:=`(abc, 1)]) :
   could not find function "remove_if_all_na"

Do you have any recommendations? The official documentation for
setRefClass is a bit thin for me but I wanted to use a tools that is going to stay. Any tip is
welcome.

Best regards,
Jeremie