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 attributes). Default namespaces must be given an explicit
name. The ns is ignored when using xml_name<-() and
xml_set_name().
# NOT RUN {x <- read_xml("<bar>123</bar>")
xml_name(x)
y <- read_xml("<bar><baz>1</baz>abc<foo /></bar>")
z <- xml_children(y)
xml_name(xml_children(y))
# }