The fractures in American society are widening, over guns, abortion, education and more. But the deepest, most traumatic fracture is surely over race. The US is post-slavery, post-segregation, but definitely not post-racism. Stephen Sackur speaks to Ibram X. Kendi, an influential writer and academic, who argues the only way to not be racist is to be actively anti-racist - a message he says children must hear. But does his approach risk intensifying America’s internal conflict?