Art students take part in a drawing competition near a portrait of former President Umaru Yar'Adua, during an art exhibition by Malami Leadership Foundation showcasing portraits of Nigeria's former presidents and heads of state in Abuja, Nigeria May 27, 2015. REUTERS/Afolabi Sotunde 

Art students take part in a competition by drawing a portrait of former President Umaru Yar'Adua, during an art exhibition by Malami Leadership Foundation showcasing portraits of Nigeria's former presidents and heads of state in Abuja, Nigeria May 27, 2015. REUTERS/Afolabi Sotunde 

Supporters of Nigeria's ailing president Umaru Yar'adua protest along a street in the capital Abuja April 27, 2010. REUTERS/Afolabi Sotunde (NIGERIA - Tags: POLITICS) 

Umaru Yar'Adua gestures as he addresses supporters during the launch of his campaign as presidential candidate of Nigeria's ruling People's Democratic Party (PDP) party during a rally in Lagos, January 27, 2007, pledging to consolidate economic reforms before thousands of flag-waving supporters. REUTERS/Akintunde Akinleye (NIGERIA) 

Umaru Yar'Adua looks at his supporters during the launch of his campaign as presidential candidate of Nigeria's ruling People's Democratic Party (PDP) party during a rally in Lagos, January 27, 2007, pledging to consolidate economic reforms before thousands of flag-waving supporters. REUTERS/George Esiri (NIGERIA) 

Nigerian President Olusegun Obasanjo (L) walks with Umaru Yar'Adua (R) during the launch of his campaign as presidential candidate of Nigeria's ruling People's Democratic Party (PDP) party during a a rally in Lagos, January 27, 2007. Yar'Adua pledged to consolidate economic reforms before thousands of flag-waving supporters. REUTERS/Akintunde Akinleye (NIGERIA) 

Umaru Yar'Adua gestures as he addresses supporters during the launch of his campaign as presidential candidate of Nigeria's ruling People's Democratic Party (PDP) party during a rally in Lagos, January 27, 2007, pledging to consolidate economic reforms before thousands of flag-waving supporters. REUTERS/George Esiri (NIGERIA)