Fix generation of protocols without precondition

benchmarks/python/execprotocol.py

@@ -42,7 +42,8 @@ else:
             protocol["transitions"][i][p] = list(map(to_string, protocol["transitions"][i][p]))
 
         protocol["predicate"] = re.sub(r'C\[([^]]*)\]', lambda match: to_string(match.group(1)), protocol["predicate"])
-        protocol["precondition"] = re.sub(r'C\[([^]]*)\]', lambda match: to_string(match.group(1)), protocol["precondition"])
+        if "precondition" in protocol:
+          protocol["precondition"] = re.sub(r'C\[([^]]*)\]', lambda match: to_string(match.group(1)), protocol["precondition"])
 
         if "statesStyle" in protocol:
           style = protocol["statesStyle"]