Mario Ruggier writes:
> The ${attr}_is class attribute naming convention is for facilitating use of
> the qp.lib.spec data validation module. I admit here it looks a bit
> cryptic, especially as it is not actually used within durus code, but I
> guess it is provided for convenience of applications that choose to use it.

It would be worthwhile to comment these various ${attr}_is attributes in
the Durus source, to make their purpose clear and avoid the kind of
misunderstanding I fell into.