How to get value of variable when using grammar file

[Yang]

Could I get the value of a variable when I use grammar file, like:
<digit>= zero | one | two | three | four | five | six | seven | eight | nine ;
<phone_num> = <digit>+;
public <call_command> = call the number <phone_num>;</phone_num></call_command></digit></phone_num></digit>

How could I get the value of the phone_num? If I have to implement it myself, which part of source code should I look into?

Thanks!

[Nickolay V. Shmyrev]

You have to implement it yourself, there is no source code to look at, you have to write it from scratch.