考虑以下sass：
$font-size: 18;
$em: $font-size;
$column: $font-size * 3; // The column-width of my grid in pixels
$gutter: $font-size * 1; // The gutter-width of my grid in pixels
$gutter-em: #{$gutter / $em}em; // The gutter-width in ems.
$column-em: #{$column / $em}em; // The column-width in ems;
$foo = $gutter-em / 2; // This results in a value like "1em/2". :(
$bar = ($gutter-em / 2); // this doesn't work either, same as above.
如何生成$foo可以正常使用的，可以在其他表达式中进一步使用的？