某程序设计语言的表达式由运算符91、92、93、标识符、(、)组成。其中91、92的优先级相同，93的优先级低于91、92，优先级相同的运算符从右向左计算，可以用括号改变运算，由这种表达式的文法可描述为()。(设E为识别符号，文法字汇表V={E,t,F,(,),θ1,θ2,θ3,i}

(88)

(86)

(87)

(85)

(83)

Artificial　intelligence (AI) ,an　interdisciplinary　field,　is　usually　regarded　as　a　branch　of　computer　science,　dealing　with　models　and　systems　for　the　performance　of　functions　generally　associated　with　human　intelligence,　such　as(71)and　learning.In　AI,　knowledge-based　system　is　an　information(72)system　that　provides　for　solving　problems　in　a　particular　domain　or　application　area　by　drawing　inferences　from　a　knowledge　base.　Moreover,　some　knowledge-based　systems　have　learning　capabilities.　Expert　system (ES) indicates　the　knowledge-based　system　that　provides　for　solving　problems　in　a　particular　domain　or　application　area　by　drawing　inferences　from　a　knowledge　base　developed　from　human(73).　Some　expert　systems　are　able　to(74)their　knowledge　base　and　develop　new　inference　rules　based　on　their　experience　with　previous　problems.　The　term　"expert　system"　is　sometimes　used(75)with　"knowledge-based　system",　but　should　be　taken　to　emphasize　expert　knowledge.

(81)

(82)

(78)

(80)