fix some bugs

修复了( var a 3 4 ) ( var b 3 4 ) ( var sum_of_squares 8 `( + ( * `,( varg
a ) `,( varg a ) ) ( * `,( varg b ) `,( varg b ) ) ) ) ( eval ( varg
sum_of_squares ) )这样的代码运行会出错的bug