"mpf_t floats have a variable precision mantissa ..."

[Bruce M. Axtens]

The question I _meant_ ask concerned the mantissa, not the exponent, and 
has lots to do with the question I asked earlier in the week regarding 
"missing" digits on the sum of two negative floats.

Given that the mantissa has a variable precision, how does one tell if 
one has overflowed the mantissa's current precision setting? Or, from 
the proactive side, how can one tell if mantissa precision overflow is 
likely?

Kind regards,
Bruce.