petri net formalism: add inhibitor arc

examples/petrinet/metamodels/mm_design.od

@@ -7,5 +7,8 @@ arc:Association (PNConnectable -> PNConnectable)
 PNPlace:Class
 PNTransition:Class
 
+# inhibitor arc
+inh_arc:Association (PNPlace -> PNTransition)
+
 :Inheritance (PNPlace -> PNConnectable)
 :Inheritance (PNTransition -> PNConnectable)

examples/petrinet/operational_semantics/r_fire_transition_nac2.od

@@ -0,0 +1,13 @@
+# A place with tokens:
+
+p:RAM_PNPlace
+ps:RAM_PNPlaceState {
+  RAM_numTokens = `get_value(this) > 0`;
+}
+:RAM_pn_of (ps -> p)
+
+# An incoming inhibitor arc from that place to our transition:
+
+t:RAM_PNTransition
+
+:RAM_inh_arc (p -> t)

examples/petrinet/renderer.py

@@ -42,5 +42,5 @@ def render_petri_net(od: ODAPI):
         src_name = od.get_name(od.get_source(arc))
         tgt_name = od.get_name(od.get_target(arc))
         dot += f"{src_name} -> {tgt_name};"
-    show_graphviz(dot, engine="dot")
+    show_graphviz(dot, engine="neato")
     return ""