A probing strategy for a given quorum set can be represented by a binary 
tree, which we call the {\bf probe tree}.
The root of the probe tree is the element that is probed first.
Unless vertex $v$ represents the last element to be probed,
the left (right) child of a vertex $v$ is the element that
is probed next when $v$ is operational (non-operational).
Thus the path from the root to a vertex $v$ corresponds to the history 
of the sequence of probes carried out so far.
