实现一个函数，完成对pascal语言中复合语句begin…end和条件语句if...then…else…配对情况的检查。其中begin…end必须成对出现，if…then…也必须成对出现，else子句可以出现也可以不出现。-Implement a function to complete if ... then ... else ... check the pairing of pascal language compound statement begin ... end and conditional statements. Which begin ... end must be paired, if ... then ... it must be paired, else clause may appear or may not appear.