Analysis of information sources in references of the Wikipedia article "Borna (duke)" in English language version.
5) Marquart, Osteuropäische u. ostasiatische Streifzöge (1903) p. XVIII, is right in identifying Porinos with Borna (Ann. r. Franc, s. a. 819). But he does not convince me that Porga is also the same, or that there was no actual foundation for the significance of the reign of Heraclius in Croatian history.