class RDoc::Markup::AttributeManager

def changed_attribute_by_name(current_set, new_set)
  current = new = 0
  current_set.each do |name|
    current |= RDoc::Markup::Attribute.bitmap_for(name)
  end
  new_set.each do |name|
    new |= RDoc::Markup::Attribute.bitmap_for(name)
  end
  change_attribute(current, new)
end