class Sanitize

def node!(node)

whitelisted or an error will be raised.
If _node_ is a `Nokogiri::XML::Document`, the `` element must be

in place.
Sanitizes the given `Nokogiri::XML::Node` and all its children, modifying it 
def node!(node)
  raise ArgumentError unless node.is_a?(Nokogiri::XML::Node)
  if node.is_a?(Nokogiri::XML::Document)
    unless @config[:elements].include?('html')
      raise Error, 'When sanitizing a document, "<html>" must be whitelisted.'
    end
  end
  node_whitelist = Set.new
  traverse(node) do |n|
    transform_node!(n, node_whitelist)
  end
  node
end