Usable and reliable is not the same thing as good design or good idea.

The fact that Windows reserves the PRN file name in every directory is probably usable and reliable to someone. That doesn't mean it's a good design for providing access to a printer device.

To test a set membership property of an object, you want a predicate function which takes the object as its only argument. For instance isnan(x).

This predicate can be efficiently implemented without relying on a bastardized equality operation.