I wrapped all of the global variables that are modified in format.scm in 
fluids and a quick test makes it appear that the new format is now threadsafe 
(the old one fails, but the new one works). 

The patch is against CVS HEAD, but it works fine with Guile 1.6.4 if the 
:replace is removed. If the patch is correct (I think it is), I don't have 
any copyright assignment papers on file with the FSF so I'd need to sign 
those.
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