Quiz – without looking (of course).A hymn to St. John the Baptist included the following line: "Ut queant laxis / resonare fibris / mira gestorum / famuli tuorum / solve polluti / labii reatum, / Sancte Ioannes" ("In order that the devout may be able to sing your wonderful deeds with all their strength, remove the guilt that pollutes their lips, O St. John.") (trans. Lanzi or O'Connell).  Guido of Arezzo (d. c. 1050) used the hymn as the basis for something very important in the world of music, which influences every student of music even today.  What is it?