class Sass::CSS

def fold_commas(root)
  prev_rule = nil
  root.children.map! do |child|
    next child unless Tree::RuleNode === child
    if prev_rule && prev_rule.children == child.children
      prev_rule.rule << ", #{child.rule}"
      next nil
    end
    fold_commas(child)
    prev_rule = child
    child
  end
  root.children.compact!
end