  An essential element of any verification technique is that of identifying and
communicating to the user, system behaviour which leads to a deviation from the
expected behaviour. Such behaviours are typically made available as long traces
of system actions which would benefit from a natural language explanation of
the trace and especially in the context of business logic level specifications.
In this paper we present a natural language generation model which can be used
to explain such traces. A key idea is that the explanation language is a CNL
that is, formally speaking, regular language susceptible transformations that
can be expressed with finite state machinery. At the same time it admits
various forms of abstraction and simplification which contribute to the
naturalness of explanations that are communicated to the user.
