class Sass::Environment

def push_frame(frame_info)
  top_of_stack = stack.last
  if top_of_stack && top_of_stack.delete(:prepared)
    top_of_stack.merge!(frame_info)
  else
    stack.push(top_of_stack = frame_info)
  end
  mixins_in_use << top_of_stack[:mixin] if top_of_stack[:mixin] && !top_of_stack[:prepared]
end