Add petrinet language

examples/petrinet/operational_semantics/r_fire_transition_lhs.od

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+t:RAM_Transition

examples/petrinet/operational_semantics/r_fire_transition_nac.od

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+# A place with no tokens:
+
+p:RAM_Place
+ps:RAM_PlaceState {
+  RAM_numTokens = `get_value(this) == 0`;
+}
+:RAM_of (ps -> p)
+
+# An incoming arc from that place to our transition:
+
+t:RAM_Transition
+
+:RAM_arc (p -> t)

examples/petrinet/operational_semantics/r_fire_transition_rhs.od

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+t:RAM_Transition {
+  condition = ```
+    # remove 1 token from every place connected with incoming arc
+    for incoming in get_incoming(this, "arc"):
+      in_place = get_source(incoming)
+      in_place_state = get_source(get_incoming(in_place, "of")[0])
+      in_num_tokens = get_slot_value(in_place_state, "numTokens")
+      set_slot_value(in_place_state, "numTokens", in_num_tokens - 1)
+      print("place", get_name(in_place_state), "now has", in_num_tokens-1, "tokens")
+
+    # add 1 token to every place connected with outgoing arc
+    for outgoing in get_outgoing(this, "arc"):
+      out_place = get_target(outgoing)
+      out_place_state = get_source(get_incoming(out_place, "of")[0])
+      out_num_tokens = get_slot_value(out_place_state, "numTokens")
+      set_slot_value(out_place_state, "numTokens", out_num_tokens + 1)
+      print("place", get_name(out_place_state), "now has", out_num_tokens+1, "tokens")
+  ```;
+}