Documentation

This is the widest word we can have before gmp will fail to allocate and bring down the whole program. According to https://gmplib.org/list-archives/gmp-bugs/2009-July/001538.html the sizes are 2^32-1 for 32-bit, and 2^37 for 64-bit, however experiments show that it's somewhere under 2^37 at least on 64-bit Mac OS X.