NS function creates namespaced IDs out of bare IDs, by joining
them using ns.sep as the delimiter. It is intended for use in Shiny
modules. See http://shiny.rstudio.com/articles/modules.html.
NS(namespace, id = NULL)
ns.sepid to be returned without a namespace, and length 2 will be
interpreted as multiple namespaces, in increasing order of specificity
(i.e. starting with the top-level namespace).id is missing, returns a function that expects an id string
as its only argument and returns that id with the namespace prepended.
character of length 1.NS function to turn a bare ID
into a namespaced one, by combining them with ns.sep in between.