 During the 1990s WNO made its <unk> debut , with a complete performance of Mozart 's <unk> , conducted by Mackerras in 1991 . The company played three short seasons at the Royal Opera House , Covent Garden in the mid @-@ 1990s , featuring Tristan und Isolde and La <unk> in 1993 , The <unk> of the Guard in 1995 , and The <unk> 's Progress and the jubilee double bill of <unk> Rusticana and Pagliacci in 1996 . 
