# File lib/nokogiri/xml/builder.rb, line 285 def [] ns @ns = @parent.namespace_definitions.find { |x| x.prefix == ns.to_s } return self if @ns @parent.ancestors.each do |a| next if a == doc @ns = a.namespace_definitions.find { |x| x.prefix == ns.to_s } return self if @ns end raise ArgumentError, "Namespace #{ns} has not been defined" end