Add theorem on the "immortality of the soul"

Sketch of the proof:
- Mentally dead state = Remains in the same H_M upon iterations of U_tau
- Form a subspace in each H_M (possibly null)
- Unreachable from a different H_M' (because of orthogonality of the subspaces and unitarity of U_tau)
- Unreachable from the same H_M (from a mentally dead + alive state) with a probability 1 (because the probability to remain in H_M from the "alive" state is bounded: It is given by a continuous function on the compact unit sphere of the "alive" subspace, the product of the probabilities to remain in H_M tends therefore to 0)

Corollary:
- There are, or will be, extra-terrestrial forms of conscious live (because terrestrial ones are going to disappear)