[plt-scheme] FFI: callbacks

I still have trouble with passing callback procedures from Scheme to C. The 
problem is when I pass a Scheme procedure as callback to a C dynamic library 
and then store it to reuse somwhere later, I accidentally get a memory 
access error. It seems to me that the callback procedure gets collected 
straight away the GC is called.
So, I think it is a fault of MzScheme FFI library.
Best regards, Ivan Boulanov