Task: Sentence Expansion
Input (English): According to Frank Stenton " The continuous history of Northumbria , and indeed of England , begins with the reign of Athelfrith ... " Among the northern Angles , Aethelfrith was the first great leader .
Output (English): The 20th-century historian Frank Stenton wrote that " the continuous history of Northumbria , and indeed of England , begins with the reign of Æthelfrith " , and that " he was the real founder of the historic Northumbrian kingdom , and he was remembered as the first great leader who had arisen among the northern Angles . "