Briefly compare the story of Chretien de Troyes with that of either Robert de Boron or Wolfram von Eschenbach. What new elements are added in or taken out?

How central to Chretien de Troyes’s story is the Grail? Is it crucial to the plot or just a picturesque but unimportant detail?
Briefly compare the story of Chretien de Troyes with that of either Robert de Boron or Wolfram von Eschenbach. What new elements are added in or taken out?
Chretien wrote other stories besides The Story of the Grail, do a search on the internet and name one and briefly describe it.