  Poly-trees are singly connected causal networks in which variables may arise
from multiple causes. This paper develops a method of recovering ply-trees from
empirically measured probability distributions of pairs of variables. The
method guarantees that, if the measured distributions are generated by a causal
process structured as a ply-tree then the topological structure of such tree
can be recovered precisely and, in addition, the causal directionality of the
branches can be determined up to the maximum extent possible. The method also
pinpoints the minimum (if any) external semantics required to determine the
causal relationships among the variables considered.
