`?=` will only set a variable if it's previously unset.
-See: [Conditional assignment](http://stylus-lang.com/docs/operators.html#conditional-assignment--)
+See: [Conditional assignment](https://stylus-lang.com/docs/operators.html#conditional-assignment--)
### Property lookup
.logo
width: w = 150
margin-left: -(w / 2)
+ // or
+ height: 80px
+ margin-top: -(@height / 2)
```
{: data-line="2,3"}
-See: [Property lookup](http://stylus-lang.com/docs/variables.html#property-lookup)
+See: [Property lookup](https://stylus-lang.com/docs/variables.html#property-lookup)
### Interpolation
-{prefix}-border-radius: 2px
```
-See: [Interpolation](http://stylus-lang.com/docs/interpolation.html)
+See: [Interpolation](https://stylus-lang.com/docs/interpolation.html)
### Color operators
```
{: data-line="3"}
-See: [lookup](http://stylus-lang.com/docs/bifs.html#lookupname)
+See: [lookup](https://stylus-lang.com/docs/bifs.html#lookupname)
Advanced features
-----------------
| `!=` | `is not` |
| `!=` | `isnt` |
-See: [Conditionals](http://stylus-lang.com/docs/functions.html#conditionals)
+See: [Conditionals](https://stylus-lang.com/docs/functions.html#conditionals)
### For loops
```
{: data-line="1"}
-See: [is defined](http://stylus-lang.com/docs/operators.html#variable-definition-is-defined)
+See: [is defined](https://stylus-lang.com/docs/operators.html#variable-definition-is-defined)
### False values
if #fff is a 'rgba' // → true
```
-See: [Instance check](http://stylus-lang.com/docs/operators.html#instance-check-is-a)
+See: [Instance check](https://stylus-lang.com/docs/operators.html#instance-check-is-a)
Built-in functions
------------------