  With the Nobel Prize attributed to Aspect, Clauser, and Zeilinger, the
international scientific community acknowledged the fundamental importance of
the experimental violation of Bell's inequalities. It is however still debated
what fails in Bell's hypotheses, leading to these inequalities, and usually
summarized as "local realism", or maybe more appropriately "classical local
realism". The most common explanation is "quantum non-locality", that remains
however fully compatible with relativistic causality; this makes wondering
whether any non-local phenomenon is really involved in these experiments. Here
we want to recapitulate another option, sometimes called "predictive
incompleteness", closely related to the idea that the usual state vector $\psi$
is incomplete indeed, as it was claimed by Einstein, Podolsky and Rosen.
However, the right way to complete $\psi$ has nothing to do with hidden
variables, but requires to specify the measurement context, as it was claimed
by Bohr. Here we will consider the simple case of two spin 1/2, or two qubits,
in order to keep the argument simple, but it does apply generally in quantum
mechanics.
