  The purpose of this note is to complete the interesting review on quantum
contextuality [1] that appeared recently. In particular we will introduce and
discuss the ideas of extracontextuality and extravalence, that allow one to
relate Kochen-Specker's and Gleason's theorems, and also to shift the emphasis
from the first to the second one. We will also argue that whereas
Kochen-Specker's is essentially a negative result (a no-go theorem), Gleason's
is a positive one since it provides a mathematical justification of Born's
rule. The link between these issues is provided by a specific quantum feature
that we call extravalence.
