default  function Cannot analysis t he

on the manual：
```less
.mixin(@value) when (ispixel(@value)) {width: @value}
.mixin(@value) when not(default())    {padding: (@value / 5)}

div-1 {
  .mixin(100px);
}

div-2 {
  /* ... */
  .mixin(100%);
}
```
result：
```css
div-1 {
  width: 100px;
  padding: 20px;
}
div-2 {
  /* ... */
}
```


but I cannot analysis the `div-2`, only `div-1 `

