class Sass::Stack

def to_s
  (frames.reverse + [nil]).each_cons(2).each_with_index.
      map do |(frame, caller), i|
    "#{i == 0 ? 'on' : 'from'} line #{frame.line}" +
      " of #{frame.filename || 'an unknown file'}" +
      (caller && caller.name ? ", in `#{caller.name}'" : "")
  end.join("\n")
end