 By the mid @-@ 1950s professional singers were cast in leading roles in most productions ; they included Walter <unk> in <unk> and La <unk> ( 1955 ) , <unk> <unk> in <unk> ( 1957 ) , Heather Harper in La traviata ( 1957 ) , and Joan Hammond in Madame Butterfly ( 1958 ) . A possibility of strengthening the professional element of the company was <unk> in 1958 , when a merger was proposed with the Carl Rosa Company , which was in financial difficulties . The proposal was not followed through and WNO continued independently while the Carl Rosa folded . 
