ob·li·ga·to
noun/ˌɒblɪˈɡɑːtəʊ/
/ˌɑːblɪˈɡɑːtəʊ/
(North American English) (also obbligato British and North American English)
(plural obligatos, obligatos)
(from Italian, music)- an important part for an instrument in a piece of music which cannot be left out
必需声部；助奏