Here is a question about this article: Kievan Rus' begins with the rule (882–912) of Prince Oleg, who extended his control from Novgorod south along the Dnieper river valley in order to protect trade from Khazar incursions from the east and moved his capital to the more strategic Kiev. Sviatoslav I (died 972) achieved the first major expansion of Kievan Rus' territorial control, fighting a war of conquest against the Khazar Empire. Vladimir the Great (980–1015) introduced Christianity with his own baptism and, by decree, that of all the inhabitants of Kiev and beyond. Kievan Rus' reached its greatest extent under Yaroslav I (1019–1054); his sons assembled and issued its first written legal code, the Rus' Justice, shortly after his death.
What is the answer to this question: What did Yaroslav sons achieve  during his time in Keivan Rus?
issued its first written legal code