In particular , the subject was suggested by a school friend that claimed to be a descendant of Madoc 's brother , Rhodri .
Southey began to write a prose version of the story in 1789 .
In particular , the subject was suggested by a school friend that claimed to be a descendant of Madoc 's brother , Rhodri , and Southey began to write a prose version of the story in 1789 .