Gödel, Escher, Bach: an Eternal Golden Braid
by Douglas Hofstadter
Rule II: Suppose you have Mx. Then you may add Mxx to your collection.⏎ What I mean by this is shown below, in a few examples.⏎⇥ From MIU, you may get MIUIU.⏎⇥ From MUM, you may get MUMUM.⏎⇥ From MU, you may get MUU.🏁