title: Sass: calling my own mixin within +background() results in "doesn't support keyword arguments"
body: <p>I'm pretty new to Sass. This is my mixin:</p>
  
  <pre><code>= foo($param1, $param2: 0, $param3: 123)
    something: $param1
    otherthing: $param2
    yetanotherthing: $param3
  </code></pre>
  
  <p>When I call it like this:</p>
  
  <pre><code>div.foo
    +foo(99, $param3: 444)
  </code></pre>
  
  <p>...it produces:</p>
  
  <pre><code>div.foo {
    something: 99;
    otherthing: 0;
    yetanotherthing: 444;
  }
  </code></pre>
  
  <p>When I call it like this:</p>
  
  <pre><code>div.foo
    +background(+foo(99, $param3: 444))
  </code></pre>
  
  <p>...it produces an ereror "Function foo doesn't support keyword arguments".</p>
  
  <p>When I call it like this:</p>
  
  <pre><code>div.foo
    +background(foo(99))
  </code></pre>
  
  <p>...it produces:</p>
  
  <pre><code>div.foo {
    background: foo(99);
  }
  </code></pre>
  
  <p>This seems weird to me. Can't I pass the result of my own mixin to another mixin? And why does something like <code>+background-image(linear-gradient(bottom, #ababab, #dadada))</code> work?</p>
  
  <p>Thanks a lot for help.</p>