Characteristic set

The characteristic set of a matroid is the set of characteristics $p$ (0 or a prime) such that the matroid is realizable over some field of characteristic $p$. It is always finite or cofinite, and it is stored as a sorted list of integers:

• contains 0: cofinite. The matroid is realizable in characteristic 0 and in every prime except the other listed primes. Non-Fano: [0, 2].
• omits 0: finite. It is realizable in exactly the listed primes. Fano: [2].
• []: not realizable over any field. Vámos: [].

We also write it in words, e.g. "characteristic 0 and all primes except 2". The set is computed from the stored realization space over $\mathbb Z$.

Full details: docs/char_set.md.