  The concept of must testing is naturally parametrised with a chosen
completeness criterion, defining the complete runs of a system. Here I employ
justness as this completeness criterion, instead of the traditional choice of
progress. The resulting must-testing preorder is incomparable with the default
one, and can be characterised as the fair failure preorder of Vogler. It also
is the coarsest precongruence preserving linear time properties when assuming
justness.
  As my system model I here employ Petri nets with read arcs. Through their
Petri net semantics, this work applies equally well to process algebras. I
provide a Petri net semantics for a standard process algebra extended with
signals; the read arcs are necessary to capture those signals.
