Eliot, all, My GSL code, with callbacks is confusing me. First, I call gsl_multifit_fdfsolver_set(). This seems to work, hitting various callbacks (apparently successfully), but then gives an error with #'bad number of arguments'. I have looked, and don't see an invalid number of arguments anywhere. I can proceed past that(??), only to crash in gsl_multiroot_fdfsolver_iterate(). The C backtrace appears below. Is there anything I might glean from it? Any ideas? Bill C stack backtrace: /home/bills/Work2012/Pharo-1.3/CogVM[0x80968b1] /home/bills/Work2012/Pharo-1.3/CogVM[0x8096a52] [0xb7720410] /usr/lib/libgsl.so.0(gsl_multifit_fdfsolver_iterate+0x37)[0x759fc5a7] /home/bills/Work2012/Pharo-1.3/libSqueakFFIPrims.so(primitiveCallout+0x4b2)[0x75c682a2] /home/bills/Work2012/Pharo-1.3/CogVM(interpret+0x3f68)[0x808dcc8] /home/bills/Work2012/Pharo-1.3/CogVM[0x808fc97] /home/bills/Work2012/Pharo-1.3/CogVM[0x8090664] /home/bills/Work2012/Pharo-1.3/CogVM(main+0x38a)[0x809774a] /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0xb7580bd6] /home/bills/Work2012/Pharo-1.3/CogVM[0x805acc1]