Is nan in (nan,) correct?

[random832 at fastmail.us]

It's been brought up on Stack Overflow that the "in" operator (on
tuples, and by my testing on dict and list, as well as dict lookup) uses
object identity as a shortcut, and returns true immediately if the
object being tested *is* an element of the container. However, the
contains operation does not specify whether object identity or equality
is to be used. In effect, the built-in container types use a hybrid
test: "a is b or a == b".

My question is, is this a *correct* implementation of the operator, or
are objects "supposed to" use a basis of equality for these tests?