According to the [doc](http://sass-lang.com/guide#topic-6), we use `@include` to call a mixin, but in my use case, I have something like this:

```
@import "components/color";

/*NavBar*/

#g-navigation{background:linear-gradient(to top,rgba(0,0,0,0), color(blue,base) 80%) !important}

```

For the mixin `color` (which is defined in `components/_color.scss`), I don't need to use `@include`, actually if I do 

```
#g-navigation{background:linear-gradient(to top,rgba(0,0,0,0), @include color(blue,base) 80%) !important}

```

the mixin will not be executed and the compiler just keep `@include` as is in the output file. So why is this? And exactly when do I need `@include` for a mixin?