According to http://man.cat-v.org/9front/2/mp mptole either take p or pp:
> Mptobe and mptole convert an mpint to a byte array. The
> former creates a big endian representation, the latter a
> little endian one. If the destination buf is not nil, it
> specifies the buffer of length blen for the result. If the
> representation is less than blen bytes, the rest of the
> buffer is zero filled. **If buf is nil**, then a buffer is
> allocated and a pointer to it is deposited in the location
> pointed to by **bufp**. Sign is ignored in these conversions,
> i.e., the byte array version is always positive.
Assert accordingly.
Giacomo Tesio
0f82a2c3c3