Pro.ID21522 TitleWhen Title链接http://10.20.2.8/oj/exercise/problem?problem_id=21522 AC0 Submit0 Ratio- 时间&空间限制描述"When" is an event driven language for machine control. It only has three statements: Set, Print, and the compound When clause. The (case insensitive) grammar is as follows PROGRAM := WHEN | PROGRAM WHEN WHEN := 'when ' EXPRESSION EOL STATEMENTS 'end when' EOL STATEMENTS := STATEMENT | STATEMENTS STATEMENT STATEMENT := PRINT | SET PRINT := 'print ' EXPRESSION_LIST EOL SET := 'set ' ASSIGNMENT_LIST EOL EXPRESSION_LIST := EXPRESSION | EXPRESSION_LIST ',' EXPRESSION ASSIGNMENT_LIST := ASSIGNMENT | ASSIGNMENT_LIST ',' ASSIGNMENT ASSIGNMENT := VARIABLE '=' EXPRESSION EXPRESSION := '(' EXPRESSION OP EXPRESSION ')' | VARIABLE | NUMBER OP := '<' | '+' | '-' | 'and' | 'or' | 'xor' VARIABLE := ' NOT_DOLLAR_STRING ' |