Add a fallback for is_known(characteristic, ::NCRing)

... to force ring implementations to provide at least one of
characteristic(R) and is_known(characteristic, R) if they want to
pass the conformance tests.

This is the only way I can see to stop rings from implementing
characteristic for their ring type but forgetting to also provide
a matching is_known(characteristic, R).

Author: fingolfin

src/NCRings.jl

@@ -229,6 +229,13 @@ function characteristic(R::NCRing)
    error("Characteristic not known")
 end
 
+# All rings are supposed to implement characteristic if they can. If they
+# can not, or only can do it with an expensive computation (e.g. a Groebner
+# basis), then instead they should implement an `is_known` method indicating
+# this. To enforce this via the ring conformance tests, we add this default
+# method for `is_known(characteristic, ::NCRing)`
+is_known(::typeof(characteristic), ::NCRing) = true
+
 ###############################################################################
 #
 #   One and zero