  Unordered data Petri nets (UDPN) are an extension of classical Petri nets
with tokens that carry data from an infinite domain and where transitions may
check equality and disequality of tokens. UDPN are well-structured, so the
coverability and termination problems are decidable, but with higher complexity
than for Petri nets. On the other hand, the problem of reachability for UDPN is
surprisingly complex, and its decidability status remains open. In this paper,
we consider the continuous reachability problem for UDPN, which can be seen as
an over-approximation of the reachability problem. Our main result is a
characterization of continuous reachability for UDPN and polynomial time
algorithm for solving it. This is a consequence of a combinatorial argument,
which shows that if continuous reachability holds then there exists a run using
only polynomially many data values.
