using
is used to load an external library.
As some of standard library is not loaded automatically, you have to load it before using it.
For example, DateTime has to be loaded before using it if you want to use DateTime
object.
using DateTime;
using
will search the source file like DateTime.kx
when it is used as using DateTime;
,
and loading it dynamically as a source code.
It means the source code will be compiled together.
using DateTime;
var a = new DateTime("2000-01-01 09:30:00");
System.println(a);
2000/01/01 09:30:00