  We extend de Finetti's (1937) notion of exchangeability to finite and
countable sequences of variables, when a subject's beliefs about them are
modelled using coherent lower previsions rather than (linear) previsions. We
prove representation theorems in both the finite and the countable case, in
terms of sampling without and with replacement, respectively. We also establish
a convergence result for sample means of exchangeable sequences. Finally, we
study and solve the problem of exchangeable natural extension: how to find the
most conservative (point-wise smallest) coherent and exchangeable lower
prevision that dominates a given lower prevision.
