  A recent paper of Edman et al. has taken a combinatorial approach to
measuring the anonymity of a threshold mix anonymous communications system.
Their paper looks at ways of matching individual messages sent to individual
messages received, irrespective of user, and determines a measure of the
anonymity provided by the system. Here we extend this approach to include in
the calculation information about how many messages were sent or received by a
user and we define a new metric that can be computed exactly and efficiently
using classical and elegant techniques from combinatorial enumeration.
