DSL Rules keyword-identifier collisions

[Nadahar]

This problem probably isn't new, but with the recent addition of conditions to DSL, quite a few new keywords have been created.

In short: In Xtext, any string explicitly defined in quotes (like 'ms') within the grammar becomes a reserved keyword for the entire language. When you use ms as a variable name in your script, the lexer prioritizes the keyword ms over a general identifier (ID).

Although "Data Type Rules" are context-sensitive and will only trigger where other circumstances match (e.g. that you're in a "condition clause"), the defined strings become global keywords.

This effectively "bans" all variable names from matching any of the defined strings. Worse yet, the exception thrown doesn't indicate that this is the cause:

g! org.eclipse.xtext.xbase [org.eclipse.xtext.xbase.jvmmodel.JvmModelAssociator] ERROR : Error calling inferrer
java.lang.NullPointerException: Cannot invoke "org.eclipse.xtext.common.types.JvmField.setStatic(boolean)" because "field" is null
	at org.openhab.core.model.rule.jvmmodel.RulesJvmModelInferrer.lambda$2(RulesJvmModelInferrer.java:115)
	at org.eclipse.xtext.xbase.lib.ObjectExtensions.operator_doubleArrow(ObjectExtensions.java:141)
	at org.openhab.core.model.rule.jvmmodel.RulesJvmModelInferrer.lambda$1(RulesJvmModelInferrer.java:121)
	at org.eclipse.xtext.xbase.lib.internal.FunctionDelegate.apply(FunctionDelegate.java:43)
	at com.google.common.collect.Lists$TransformingRandomAccessList$1.transform(Lists.java:654)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:51)
	at org.eclipse.xtext.xbase.jvmmodel.JvmTypesBuilder.operator_add(JvmTypesBuilder.java:139)
	at org.openhab.core.model.rule.jvmmodel.RulesJvmModelInferrer.lambda$0(RulesJvmModelInferrer.java:124)
	at org.eclipse.xtext.xbase.jvmmodel.JvmModelAssociator$1.run(JvmModelAssociator.java:398)
	at org.eclipse.xtext.xbase.jvmmodel.JvmModelAssociator.installDerivedState(JvmModelAssociator.java:408)
	at org.eclipse.xtext.resource.DerivedStateAwareResource.installDerivedState(DerivedStateAwareResource.java:244)
	at org.eclipse.xtext.xbase.resource.BatchLinkableResource.getContents(BatchLinkableResource.java:149)
	at org.openhab.core.model.core.internal.ModelRepositoryImpl.validateModel(ModelRepositoryImpl.java:326)
	at org.openhab.core.model.core.internal.ModelRepositoryImpl.addOrRefreshModel(ModelRepositoryImpl.java:121)
	at org.openhab.core.model.core.internal.ModelRepositoryImpl.addOrRefreshModel(ModelRepositoryImpl.java:109)
	at org.openhab.core.model.core.internal.folder.FolderObserver.checkPath(FolderObserver.java:285)
	at org.openhab.core.model.core.internal.folder.FolderObserver.processIgnoredPaths(FolderObserver.java:194)
	at org.openhab.core.model.core.internal.folder.FolderObserver.addModelParser(FolderObserver.java:103)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.apache.felix.scr.impl.inject.methods.BaseMethod.invokeMethod(BaseMethod.java:245)
	at org.apache.felix.scr.impl.inject.methods.BaseMethod.access$500(BaseMethod.java:41)
	at org.apache.felix.scr.impl.inject.methods.BaseMethod$Resolved.invoke(BaseMethod.java:687)
	at org.apache.felix.scr.impl.inject.methods.BaseMethod.invoke(BaseMethod.java:531)
	at org.apache.felix.scr.impl.inject.methods.BindMethod.invoke(BindMethod.java:42)
	at org.apache.felix.scr.impl.manager.DependencyManager.doInvokeBindMethod(DependencyManager.java:2086)
	at org.apache.felix.scr.impl.manager.DependencyManager.invokeBindMethod(DependencyManager.java:2061)
	at org.apache.felix.scr.impl.manager.SingleComponentManager.invokeBindMethod(SingleComponentManager.java:443)
	at org.apache.felix.scr.impl.manager.DependencyManager$MultipleDynamicCustomizer.addedService(DependencyManager.java:336)
	at org.apache.felix.scr.impl.manager.DependencyManager$MultipleDynamicCustomizer.addedService(DependencyManager.java:304)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184)
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116)
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915)
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1274)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1225)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184)
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116)
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915)
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1274)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1225)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184)
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116)
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915)
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1274)
	at org.apache.felix.scr.impl.manager.DependencyManager$SingleStaticCustomizer.addedService(DependencyManager.java:1225)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1232)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.customizerAdded(ServiceTracker.java:1152)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.trackAdding(ServiceTracker.java:959)
	at org.apache.felix.scr.impl.manager.ServiceTracker$AbstractTracked.track(ServiceTracker.java:895)
	at org.apache.felix.scr.impl.manager.ServiceTracker$Tracked.serviceChanged(ServiceTracker.java:1184)
	at org.apache.felix.scr.impl.BundleComponentActivator$ListenerInfo.serviceChanged(BundleComponentActivator.java:116)
	at org.eclipse.osgi.internal.serviceregistry.FilteredServiceListener.serviceChanged(FilteredServiceListener.java:123)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:961)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEventPrivileged(ServiceRegistry.java:937)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.publishServiceEvent(ServiceRegistry.java:874)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistrationImpl.register(ServiceRegistrationImpl.java:141)
	at org.eclipse.osgi.internal.serviceregistry.ServiceRegistry.registerService(ServiceRegistry.java:262)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.registerService(BundleContextImpl.java:500)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:929)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager$3.register(AbstractComponentManager.java:915)
	at org.apache.felix.scr.impl.manager.RegistrationManager.changeRegistration(RegistrationManager.java:133)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.registerService(AbstractComponentManager.java:984)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.activateInternal(AbstractComponentManager.java:752)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.enableInternal(AbstractComponentManager.java:674)
	at org.apache.felix.scr.impl.manager.AbstractComponentManager.enable(AbstractComponentManager.java:437)
	at org.apache.felix.scr.impl.manager.ConfigurableComponentHolder.enableComponents(ConfigurableComponentHolder.java:671)
	at org.apache.felix.scr.impl.BundleComponentActivator.initialEnable(BundleComponentActivator.java:310)
	at org.apache.felix.scr.impl.Activator.loadComponents(Activator.java:593)
	at org.apache.felix.scr.impl.Activator.access$200(Activator.java:74)
	at org.apache.felix.scr.impl.Activator$ScrExtension.start(Activator.java:460)
	at org.apache.felix.scr.impl.AbstractExtender.createExtension(AbstractExtender.java:196)
	at org.apache.felix.scr.impl.AbstractExtender.modifiedBundle(AbstractExtender.java:169)
	at org.apache.felix.scr.impl.AbstractExtender.modifiedBundle(AbstractExtender.java:49)
	at org.osgi.util.tracker.BundleTracker$Tracked.customizerModified(BundleTracker.java:488)
	at org.osgi.util.tracker.BundleTracker$Tracked.customizerModified(BundleTracker.java:1)
	at org.osgi.util.tracker.AbstractTracked.track(AbstractTracked.java:232)
	at org.osgi.util.tracker.BundleTracker$Tracked.bundleChanged(BundleTracker.java:450)
	at org.eclipse.osgi.internal.framework.BundleContextImpl.dispatchEvent(BundleContextImpl.java:949)
	at org.eclipse.osgi.framework.eventmgr.EventManager.dispatchEvent(EventManager.java:234)
	at org.eclipse.osgi.framework.eventmgr.ListenerQueue.dispatchEventSynchronous(ListenerQueue.java:151)
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEventPrivileged(EquinoxEventPublisher.java:229)
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:138)
	at org.eclipse.osgi.internal.framework.EquinoxEventPublisher.publishBundleEvent(EquinoxEventPublisher.java:130)
	at org.eclipse.osgi.internal.framework.EquinoxContainerAdaptor.publishModuleEvent(EquinoxContainerAdaptor.java:217)
	at org.eclipse.osgi.container.Module.publishEvent(Module.java:499)
	at org.eclipse.osgi.container.Module.start(Module.java:486)
	at org.eclipse.osgi.internal.framework.EquinoxBundle.start(EquinoxBundle.java:445)
	at aQute.launcher.Launcher.start(Launcher.java:699)
	at aQute.launcher.Launcher.startBundles(Launcher.java:679)
	at aQute.launcher.Launcher.activate(Launcher.java:585)
	at aQute.launcher.Launcher.launch(Launcher.java:403)
	at aQute.launcher.Launcher.run(Launcher.java:185)
	at aQute.launcher.Launcher.main(Launcher.java:161)
	at aQute.launcher.pre.EmbeddedLauncher.executeWithRunPath(EmbeddedLauncher.java:170)
	at aQute.launcher.pre.EmbeddedLauncher.findAndExecute(EmbeddedLauncher.java:135)
	at aQute.launcher.pre.EmbeddedLauncher.main(EmbeddedLauncher.java:52)

The recent addition of conditions have added a number of new such "keywords" that will break existing rules.

[Nadahar]

The conflict can be avoided by prefixing the variable name with ^, so that you use ^ms instead of ms (but simply using another name would also "solve it"). That is still a major inconvenience.

I can see two possible ways to handle it:

1. Redesign all conditions to use less generic terms, combine them (but only if instead of but only if) and generally make conflicts as unlikely as possible. Conflicts with existing trigger terms aren't new, so users should already have avoided these.

2. Use a "hack" I've found, where you also define these terms as possible variable names. That seems to make it work, but I don't know if it has other implications. By adding this definition, I've gotten rules with "conflicting variable names" to work:

@Override
ValidID:
    ID | 'ms' | 'is' | 'a' | 'to' | 'from' | 'and' | 'or' | 'start' | 'between' | 'executions' | 'noon' | 'midnight'
;

More terms would have to be added, but with this definition in Rules.xtext, I've successfully used both ms and a as variable names.

Solution 2) is the most flexible, it doesn't restrict the syntax from being "natural language like". I am however worried that it might have side effects that we'll only discover later. If 2) is selected, I think it should be thoroughly tested.

[lolodomo]

@Nadahar :great finding.
I would suggest to combine both of your options.
For example, we can probably remove "a" as keyword.

[dilyanpalauzov]

If the ValidID redefinition contains 'uid' | 'rule', does it allow naming variables as uid or rule?

With openHAB 5.1. global var then = 7 produces

2026-04-18 10:21:02.045 [INFO ] [el.core.internal.ModelRepositoryImpl] - Loading DSL model 'a.rules'
2026-04-18 10:21:02.128 [ERROR] [xt.xbase.jvmmodel.JvmModelAssociator] - Error calling inferrer
java.lang.NullPointerException: Cannot invoke "org.eclipse.xtext.common.types.JvmField.setStatic(boolean)" because "field" is null
at org.openhab.core.model.rule.jvmmodel.RulesJvmModelInferrer.lambda$2(RulesJvmModelInferrer.java:118) ~[?:?]

but var then = 7 within a rule prints no stack trace:

2026-04-18 10:22:34.931 [INFO ] [el.core.internal.ModelRepositoryImpl] - Loading DSL model 'a.rules'
2026-04-18 10:22:35.042 [WARN ] [el.core.internal.ModelRepositoryImpl] - DSL model 'a.rules' has errors, therefore ignoring it: [6,9]: no viable alternative at input 'then'
2026-04-18 10:22:35.043 [INFO ] [el.core.internal.ModelRepositoryImpl] - Unloading DSL model 'a.rules'

I suggest changing in ('and' conditions+=Condition)*)? and 'Time' 'is' 'between' start=TIME 'and' end=TIME the and with &&.

[Nadahar]

@dilyanpalauzov I don't understand what you mean. We should only use this to make nested, context-sensitive keywords work as variables. "Actual keywords", especially at the top level, should not be in this list.

The problem is for conditions but only if. It consists of such words, but is at the same time a "top level" keyword. I think it would be better to make it one keyword instead of a combination of 3, it would remove this dilemma (and yes, I know that Eclipse issues warnings if you have space in a keyword, but there are plenty of them there already, and it doesn't seem to cause any issues).

[dilyanpalauzov]

I mean, if it is possible to allow variables named then, rule, uid, is, a, without creating ambiguity, then these variable names should be allowed.

[Nadahar]

I can't say how well it will work, I consider it a kind of "hack" to stop the lexer from deciding that they are keywords. From what I read, you can get into situations where you confuse the parser by doing this, so I don't think "top level" keywords like then, rule etc. should be in the list. But, uid I think should be acceptable. It only applies as a keyword when immediately following the rule name, which isn't a place you can use a variable, so there should be no ambiguity.

But, in the end, I can't predict what kind of problems using this may cause, which is why I recommend heavy testing.

[Nadahar]

@rkoshak I think you have talked about this in the past, that it's "an annoyance" in DSL that some words are "magically defunct" and will cause seemingly unrelated exceptions if used? Or am I confused?

[dilyanpalauzov]

From what I read, you can get into situations where you confuse the parser by doing this, so I don't think "top level" keywords like then, rule etc. should be in the list.

My understanding is that if the grammar is ambiguous, then Xtext produces compile-time warning. Compile-time means when Rules.xtext is read, not when the DSL Rules are read.

[Nadahar]

My understanding is that if the grammar is ambiguous, then Xtext produces compile-time warning. Compile-time means when Rules.xtext is read, not when the DSL Rules are read.

I don't know anything for sure, but it's clear that the logic isn't always "waterproof". I read that you could get into situations where the parser would bug because of ambiguity, which sounds reasonable to me from what I know about Xtext/tend/base, so I think it would be smart to avoid the "most obvious" potential conficts.

Xtext is one thing alone, but we have the extra challenge that Xtend/base is used on top here, interpreting it a programming language, which makes everything more complex than if you use Xbase to simply define a file format.

I can't see that it's that important to be allowed to use rule, then, when and end as variable names that it's worth the risk.

[rkoshak]

@rkoshak I think you have talked about this in the past, that it's "an annoyance" in DSL that some words are "magically defunct" and will cause seemingly unrelated exceptions if used?

I didn't think so. I've complained about the way it handles determining the types of Objects at runtime so the following line works:

if( 7 > MyItem.state)

but this line will throw a null pointer exceptions.

if(MyItem.state < 7)

The first argument sets the "type" for the operation. In the error case, the type is State and there is no < operator that works between State and Number. The interpreter never looks down the class hierarchy of the first operand to see it's actually a DecimalType which is a Number.

But I'm OK with reserved words in general so long as it's clear what they are. I didn't think there are any in particular in DSL that give me concern. I would assume rule, than, when, and end to be reserved.

[Nadahar]

But I'm OK with reserved words in general so long as it's clear what they are. I didn't think there are any in particular in DSL that give me concern. I would assume rule, than, when, and end to be reserved.

Yes, those are fine, but there are currently many more - and the error you get about a NPE doesn't in any way indicate what the problem is, so there's really no way for a user to even understand that the problem is that it's a reserved word.

Basically, anything you find in this file:

https://github.com/openhab/openhab-core/blob/main/bundles/org.openhab.core.model.rule/src/org/openhab/core/model/rule/Rules.xtext

..that is enclosed in single quotes, are "reserved words", which include a, is, to, from and many others. Sorry to drag you in if I misremembered then, I'm sure that somebody complained that some words just "magically" made the rules fail, and I just wanted to let the person know that there might be a way to get rid of this. But, if it wasn't you, I have no idea who 😉

[lolodomo]

I will consider these discussions in the change I am starting to implement.