class REXML::Element

e.namespaces # => {"xmlns"=>"bar", "baz"=>"bat"}
e.add_namespace('baz', 'bat')

adds a namespace using both arguments:
With both arguments +prefix+ and +uri+ given,

e.namespaces # => {"xmlns"=>"bar"}
e.add_namespace('bar')
e = REXML::Element.new('foo')

adds a namespace using the given +prefix+ and the namespace URI:
With the single argument +prefix+,

Adds a namespace to the element; returns +self+.

add_namespace(prefix, uri = nil) -> self
:call-seq:

def add_namespace( prefix, uri=nil )
  unless uri
    @attributes["xmlns"] = prefix
  else
    prefix = "xmlns:#{prefix}" unless prefix =~ /^xmlns:/
    @attributes[ prefix ] = uri
  end
  self
end