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pub use self::gmp_classgroup::do_compute;
Exports obj to v as an array of 2’s complement, big-endian
bytes. If v is too small to hold the result, returns Err(s),
where s is the size needed to hold the exported version of obj.
Unmarshals a buffer to an Mpz. buf is interpreted as a 2’s complement,
big-endian integer. If the buffer is empty, zero is returned.