  This paper deals with the problem of enforcing modular diagnosability for
discrete-event systems that don't satisfy this property by their natural
modularity. We introduce an approach to achieve this property combining
existing modules into new virtual modules. An underlining mathematical problem
is to find a partition of a set, such that the partition satisfies the required
property. The time complexity of such problem is very high. To overcome it, the
paper introduces a structural analysis of the system's modules. In the analysis
we focus on the case when the modules participate in diagnosis with their
observations, rather then the case when indistinguishable observations are
blocked due to concurrency.
