diff --git a/plugins/org.etsi.mts.tdl.model/model/tdl-constraints.ocl b/plugins/org.etsi.mts.tdl.model/model/tdl-constraints.ocl
index dd5183042bbd4a3023fdbeb8a82b1371f289c168..246dc6888aa537902ac00f657f5749c19d94e770 100644
--- a/plugins/org.etsi.mts.tdl.model/model/tdl-constraints.ocl
+++ b/plugins/org.etsi.mts.tdl.model/model/tdl-constraints.ocl
@@ -695,49 +695,54 @@ context AlternativeBehaviour
 
   -- First behaviour of 'Block's
   inv FirstBlockBehaviour ('Each block of an \'AlternativeBehaviour\' shall start with a tester-input event. ' + self.toString()):
-        self.block->forAll(b | b.behaviour->first().isTesterInputEvent())
+        self.block->forAll(b | b.behaviour->isEmpty() or b.behaviour->first().isTesterInputEvent())
   
 
   -- Same component if locally ordered
   inv AlternativeBlocksComponent ('If the containing \'TestDescription\' is locally ordered then all \'Block\'s shall start with a tester-input event of the same \'ComponentInstance\'. ' + self.toString()):
-        let initial = self.block.behaviour->first() in
+        let initials = self.block->reject(b | b.behaviour->first()->isEmpty() or b.behaviour->first().oclIsKindOf(OptionalBehaviour)).behaviour->first()
+          in
           Set{}
-          ->including(initial->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
-          ->including(initial->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
-          -> including(initial->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance)
+          ->union(initials->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
+          ->union(initials->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
+          ->union(initials->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance)
           ->size() = 1 or not self.getParentTestDescription().isLocallyOrdered
   
 
   -- Tester participating in locally ordered case
   inv AlternativeBehaviourParticipation ('If the \'AlternativeBehaviour\' is contained in a locally ordered \'TestDescription\' then no other tester \'ComponentInstance\' shall participate in any block than the target of the first tester-input event and \'ComponentInstance\'s participating in blocks of contained \'OptionalBehaviour\'s. ' + self.toString()):
-        let initial = self.block.behaviour->first(),
+        let initials = self.block->reject(b | b.behaviour->first()->isEmpty() or b.behaviour->first().oclIsKindOf(OptionalBehaviour)).behaviour->first(),
           targetComponent = Set{}
-          ->including(initial->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
-          ->including(initial->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
-          -> including(initial->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance),
+          ->union(initials->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
+          ->union(initials->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
+          -> union(initials->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance),
           nonOptionalBlocks = self.block->closure(
               b | b.behaviour->reject(oclIsKindOf(OptionalBehaviour))
                   ->select(oclIsKindOf(SingleCombinedBehaviour)).oclAsType(SingleCombinedBehaviour).block
-              ->union(b.behaviour->reject(oclIsKindOf(OptionalBehaviour))
+              ->union(b.behaviour
                   ->select(oclIsKindOf(MultipleCombinedBehaviour)).oclAsType(MultipleCombinedBehaviour).block)
           )
           in
-          targetComponent->includesAll(
+            targetComponent->includesAll(
               nonOptionalBlocks.getParticipatingComponents()->reject(c | c.role = ComponentInstanceRole::SUT))
-          or not self.getParentTestDescription().isLocallyOrdered
+            or not self.getParentTestDescription().isLocallyOrdered
   
 
   -- OptionalBehaviour in locally ordered case
   inv OptionalAlternativeBehaviour ('A block of an \'AlternativeBehaviour\' if the containing \'TestDescription\' is locally ordered, shall only contain \'OptionalBehaviour\'(s) whose source \'ComponentInstance\' is the same as the target of the first tester-input event of that \'Block\'. ' + self.toString()):
-        let initial = self.block.behaviour->first(),
+        let initials = self.block->reject(b | b.behaviour->first()->isEmpty() or b.behaviour->first().oclIsKindOf(OptionalBehaviour)).behaviour->first(),
           targetComponent = Set{}
-          ->including(initial->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
-          ->including(initial->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
-          -> including(initial->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance)
+          ->union(initials->select(oclIsKindOf(Interaction)).oclAsType(Interaction).target.targetGate.component)
+          ->union(initials->select(oclIsKindOf(Quiescence)).oclAsType(Quiescence).componentInstance)
+          -> union(initials->select(oclIsKindOf(TimeOut)).oclAsType(TimeOut).componentInstance),
+          optionalBlocks = self.block->closure(
+            b | b.behaviour->select(oclIsKindOf(SingleCombinedBehaviour)).oclAsType(SingleCombinedBehaviour).block
+            ->union(b.behaviour->select(oclIsKindOf(MultipleCombinedBehaviour)).oclAsType(MultipleCombinedBehaviour).block)
+          )->select(oclIsKindOf(OptionalBehaviour))
           in
-          self.block.behaviour->select(oclIsKindOf(OptionalBehaviour)) .oclAsType(OptionalBehaviour).block
-              ->first().oclAsType(Interaction).sourceGate.component->forAll(c | targetComponent->includes(c))
-          or not self.getParentTestDescription().isLocallyOrdered
+            optionalBlocks->isEmpty()
+            or optionalBlocks->first().oclAsType(Interaction).sourceGate.component->forAll(c | targetComponent->includes(c))
+            or not self.getParentTestDescription().isLocallyOrdered