Felec could be equated with Felix, a supposed early king of either Cornwall or Lyonesse according to the Prose Tristan (c. 1235) and later Italian Arthurian romances, but this reference is very late.
In the Prose Tristan, Mark's character deteriorates from a sympathetic cuckold to a downright villain.
He becomes a Knight of the Round Table (taking Morholt's old seat) and embarks on the Quest for the Holy Grail before abandoning the idea to stay with Iseult at Lancelot's castle.
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