A `define` is similiar to a preprocessor `#define` in **C**, the value gets
substituted into the expression. Subsitution is only within template brackets.
Substitution is done during the compile stage of the template.

#### Example 1
```
{% define PC = [self stringByReplacingOccurrencesOfString:@"S04"
                                               withString:@"***"] %}

{% filter PC %}
   VfL vs. S04 = 4:0
{% endfilter %}
```

`define` is a command, that can not appear in multi-line statements.
`define` is something that is not in TWIG.