I was listening to an old favorite, the [Helsinki Comlaints Choir](https://youtu.be/ATXV3DzKv68?si=rhBAjo_S0HMjNIoZ), this evening, and it suddenly occurred to me that I have no idea what a “reference number” (time: [3m05s](https://youtu.be/ATXV3DzKv68?t=3m05s)) is.
by TychaBrahe