Commentary Par XV 51

That is to say, the words in this 'book' are unchanging, unlike those in human manuscripts, where scribes variously blot, erase, add to, and cross out previous texts.  Cf. [Par XVII 37-39].  And see Torraca (DDP Torraca.Par.XV.49-51) for a reflection of this verse in the opening line of Dante's first Eclogue to Giovanni del Virgilio: 'Vidimus in nigris albo patiente lituris...' (In letters black, upon receptive white, I saw...).