# File lib/sass/environment.rb, line 55
    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]
      files_in_use << top_of_stack[:filename] if top_of_stack[:filename]
    end