Some algorithms require a quadratic nonresidue in a given field. These
are computed lazily: The first time a quadratic nonresidue is requested, one is
generated at random, using the same source of random bits as other PBC random
functions. [Which reminds me, should I get rid of the `nqr`

field and instead
have it as part of the `data`

field in struct field_s?]

In `fieldquadratic.c`

, a quadratic field extension is constructed with a square
root of this randomly generated quadratic nonresidue in the base field. Thus
for a nondeterminstic source of random bits, the same field may be constructed
differently on different runs.

To construct the same field the same way every time, one must record the
quadratic nonresidue generated from one run, and call `field_set_nqr()`

every
time this particular construction of a quadratic field extension is desired.
Another use for this function is to save time by setting the quadratic
nonresidue to some precomputed value.

Similarly, for higher degree extensions, a random irreducible polynomial may be chosen to construct it, but this must be recorded if the same construction is later required.

This happens behind the scenes in PBC.