  We call "proteons" -- from the ever-changing greek sea-god
$\Pi\rho\omega\tau\epsilon\upsilon\varsigma$ -- composite particles made of two
fermions. Among them, are the semiconductor excitons, but also various atoms
and molecules, like the giant molecules made of two $^{40}$K or $^6$Li atoms
which have recently Bose condensed. In addition to their indistinguishability,
these composite particles are ``ever-changing'' in the sense that there is no
way to know with which fermions they are precisely made. As direct
consequences, (i) the proteons are not true bosons, (ii) the basis made with
proteon states is \emph{overcomplete}. In spite of these difficulties, these
proteons do have a nice closure relation, unexpected at first, \emph{different
from the boson one} and which makes the bosonization procedures used up to now
to treat many-body effects between composite bosons, rather questionable, due
to possibly incorrect sum rules resulting from it. This closure relation in
particular explains, in a neat way, the surprising factor 1/2 between the
inverse lifetime and the sum of scattering rates which exists for exact
excitons but not for boson excitons, as we have recently shown.
