Optionally, a named vector giving prefix-url pairs, as produced
by xml_ns. If provided, all names will be explicitly
qualified with the ns prefix, i.e. if the element bar is defined
in namespace foo, it will be called foo:bar. (And
similarly for atttributes). Default namespaces must be given an explicit
name. The ns is ignored when using xml_name<- and
xml_set_name.