How to watch a variable?

[Ye Wenbin]

Hi,
I found an anony problem. I customize set outline-regexp, but
the variable is change when start up. I don't know which part
of my .emacs changed the value. So is it possible to add some
hook when change the variable?
I found the variable truncate-lines is different. When evaluate
(setq truncate-lines t), it will take affect without calling other
functions. How to make the outline-regexp like it?

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Best regards,
Ye Wenbin