[plt-scheme] Synchronizing

I would like to make a procedure, that may be called from many threads, 
but if it is called from two threads at the same time it will block the 
second one until the first one finishes, and then run it.

I want this because I have many drawing procedures that merely add 
something to a list, and appearantly some things get deleted because the 
procedures are called at the same time.

Is there a simple solution to this?

Katsmall the Wise