#define - Assigns a block of VTL to a reference

```
import airspeed
d = """
#define( $hello ) Hello $who #end 
#set( $who = "World!") $hello 
"""
t = airspeed.Template(d)
o = t.merge({   })
print o
```

http://velocity.apache.org/engine/devel/vtl-reference-guide.html
