The rock of Gibraltar, the great pyramid of Cheops, the Pope calling for peace at Christmas. These are the constants of life. Good.
But for some things, their lease hath all too short a date. Your email address. That credential that proves who you are. They go stale — your old friends can't find you, and you can't authenticate to get what you want. Not good.
Well those days are over. Try an Invariant Solution.