[plt-scheme] Thread synchronization for dummies

Hi,

Suppose I have a function FOO with side effects (filesystem access)  
and would like to ensure that if this function is called by two  
different threads A and B at the same time, either A executes FOO  
entirely and then B executes FOO entirely or vice versa. In other  
words, I'm looking for something like "synchronized" does in Java.

How to?

Best regards,

Erich