guttentag writes "The New York Times has published a large collection of law enforcement training documents obtained by the ACLU. The documents describe in detail what kind of information can be obtained from cell phones and cell phone carriers, and how to obtain it. The 189-page PDF also contains dozens of invoices from the major carriers for their services to law enforcement that describe the fees for those services."
We warn the reader in advance that the proof presented here depends on a
clever but highly unmotivated trick.
-- Howard Anton, "Elementary Linear Algebra"