f语句的无二义性文法定义 Stmt → Matched stmt I Unmatched stmt Matched stmt ife then matched stmt else Matched stmt I other Unmatched stmt ife then stmt ife then matched stmt else unmatched stmtIf语句的无二义性文法定义 Stmt → Matched_stmt | Unmatched_stmt Matched_stmt → if E then Matched_stmt else Matched_stmt | other Unmatched_stmt → if E then Stmt | if E then Matched_stmt else Unmatched_stmt