docs: private ExqlUnit compileBlock() {
  
          // 查找匹配的  {...}
          String group = findBrace(BLOCK_LEFT, BLOCK_RIGHT);
          if (group != null) {
  
              // 编译  {...} 内部的子句
              ExqlCompiler compiler = new ExqlCompiler(group);
              return compiler.compileUnit();
          }
  
          return null; // 没有  {...} 匹配
      }