Thiagarajan's finite 1-safe Petri-net characterization conjecture

Let ${\mathcal E}$ be an event structure, and let ${\mathcal E}_N$ denote the event-structure unfolding arising from a finite 1-safe Petri net $N$. Thiagarajan's finite 1-safe Petri-net conjecture. An event structure ${\mathcal E}$ is isomorphic to ${\mathcal E}_N$ for some finite 1-safe Petri net $N$ if and only if ${\mathcal E}$ is regular. The supplied text does not state whether this conjecture has been resolved.