EntityModel returns system::EntityModel:
	'entities for module' 
	module = [system::Module | QualifiedName]
	'version' version = STRING 
	(entities += Entity)+
;

Entity returns modulob::Entity:
	(annotations+=Annotation)*
	(isAbstract?='abstract')? 'entity' name=ID (':' super=[modulob::Entity])?'{'
		(features+=Feature)*
	'}'
;

entities for module iPhoneApp.Reporting version "1.0"

entity AnEntity {
	
}

java.lang.RuntimeException: Could not serialize EObject via backtracking.
Constraint: (null) module=[Module|QualifiedName] version=STRING entities+=Entity+ (null)
Semantic Object: EntityModel
Context: EntityModel
	at org.eclipse.xtext.serializer.diagnostic.ISerializationDiagnostic$ExceptionThrowingAcceptor.accept(ISerializationDiagnostic.java:66)
	at org.eclipse.xtext.serializer.sequencer.BacktrackingSemanticSequencer.createSequence(BacktrackingSemanticSequencer.java:418)
	at de.dubmas.modulob.serializer.AbstractDataDslSemanticSequencer.sequence_EntityModel(AbstractDataDslSemanticSequencer.java:176)
	at de.dubmas.modulob.serializer.AbstractDataDslSemanticSequencer.createSequence(AbstractDataDslSemanticSequencer.java:107)
	at org.eclipse.xtext.serializer.impl.Serializer.serialize(Serializer.java:76)
	at org.eclipse.xtext.serializer.impl.Serializer.serialize(Serializer.java:84)
	at org.eclipse.xtext.serializer.impl.Serializer.serialize(Serializer.java:106)
	at org.eclipse.xtext.resource.XtextResource.doSave(XtextResource.java:286)