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A probabilistic ancestral sequence (PAS) is like a protein sequence
where each position, instead of an amino acid, has a vector
describing the probability
of each amino acid being in that position.
A PAS is normally associated to an EPT and to a particular place
in this tree.
It is normally most interesting to compute the PAS at divergence
points or at the root of the EPT.
E.g. find the PAS for the common ancestor of all mammals.