The code can be denoted:
This one line does:

1. Declare numa
2. Declare numb
3. 21 → numb
4. 21 → numa

To understand this, let's use print function with the parameter 21

This line of code yields the value 21. Then this value can be assigned to a variable:

num gets the result of the statement print 21, which is 21.

In Tyl, the result of a statement is usually its own value or its returned value:

The result of the assignment will be the value 21.

For any statement, prepending it with print will print its result. Another option is to use the debug print function, which logs the result in a debug file.

So we can write also:

The system will print 21, which is the result of the statement num 21.

So in the statement:

The system first declares numa. Then the statement numb 21, declares numb, assignes it the value 21, which yields the result 21. The result 21 is then assigned to numa.