Tolong jelaskan mengenai Sumber dari Vratislav I

Ancestral Roots of Certain American Colonists Who Came to America Before 1700 by Frederick Lewis Weis; Line 244-7

The Plantagenet Ancestry by William Henry Turton, Page 85

Royal Highness, Ancestry of the Royal Child by Sir Iain Moncreiffe, Pages 64–65

Penguasa abad ke-10 di Eropa