The Mishnah Shabbat 6:1 states:
במה אשה יוצאה ובמה אינה יוצאה. ... ולא בעיר של זהב.
The commentaries explain that the "City of Gold" is Yerushalayim.
Why is it forbidden to go out with Yerushalayim Shel Zahav on Shabbat? Obviously playing the song on an MP3 player or live instrument is forbidden, but this seems to imply that even walking around with the lyrics or sheet music is forbidden. (How about singing? Is that allowed?)
What does the Mishnah have against the song?