No meaningful hash function can be defined for elements which overwrite the
operator == like the p-adics do since the contract on __hash__ would be broken:
elements which compare equal would have different hash values. This would break
caches and lead to subtle bugs. Still such elements should be cacheable for
performance. As a workaround, such non-hashable elements can define a
_cache_key which uniquely identifies them, and so fixes the "broken" ==
operator.