“The opera, Harry Somers’s Louis Riel, tells the story of Riel, who led two 19th-century uprisings against the young nation of Canada, helped found Manitoba and was hanged for treason.” Missing from the score, and from its early stagings around Canada’s centennial, were the voices of the country’s First Nations. The Canadian Opera Company’s new production (for Canada’s 150th anniversary) has found a poetic way to address that absence.