For a black American president on a tour of Africa, it seemed an obvious opportunity for a speech about the evils of slavery. When Barack Obama visited Senegal on Thursday, he headed for Gorée Island, where a crumbling fort is said to have been the key departure point for millions of slaves shipped across the Atlantic. Yet as the president toured the 18th century building and later spoke of how the visit had allowed him to "fully appreciate the magnitude of the slave trade", historians pointed out that it was probably never used for that purpose at all.