Personas 3.0 SP02: How to call a lib function defined in another script of other screen?

Hi,

There is a function, myLib(), defined in a script A in screen A, and I want to call this function from script B in screen B.

I experienced that sometimes personas works, but sometimes it reported that the lib function is undefined. Why?

When I use the script id, do I have to add screen number or tcode as well?

I also experienced that some other event was triggered in such way.

Thanks in advance.

Dong