Lazy mpz allocation

[Torbjörn Granlund]

Marc Glisse <marc.glisse at inria.fr> writes:

  For mpq, the attached seems sufficient to pass make check. Of course,
  without a copy-on-write mechanism for the 1 in the denominator, the
  gains are not comparable to the mpz case, it is more for consistency.

I forgot about that we need to have an explicit denominator.
We could surely point that to some static read-only data, but that would
of course incur a cost when "re-allocating".

-- 
Torbjörn
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