  We study the following summarization problem: given a parallel composition
A=A1||...||An of labelled transition systems communicating with the environment
through a distinguished component Ai, efficiently compute a summary Si such
that E||A and E||Si are trace-equivalent for every environment E. While Si can
be computed using elementary automata theory, the resulting algorithm suffers
from the state-explosion problem. We present a new, simple but subtle algorithm
based on net unfoldings, a partial-order semantics, give some experimental
results using an implementation on top of MOLE, and show that our algorithm can
handle divergences and compute weighted summaries with minor modifications.
