Assertion error when saving proof files

I have a problem when saving a proof file. It seems like there is an asserting error in the Event-B Core. Could you please have a look at it. I do not know if
this is reproduce-able.
Thanks,
Son

!SESSION 2008-04-17 11:29:23.927 -----------------------------------------------
eclipse.buildId=unknown
java.version=1.5.0_14
java.vendor=Sun Microsystems Inc.
BootLoader constants: OS=linux, ARCH=x86, WS=gtk, NL=vi_VN
Framework arguments: -product org.rodinp.platform.product
Command-line arguments: -product org.rodinp.platform.product -data /home/htson/work/rodin-workspace -dev file:/home/htson/work/Rodin-Europa-Editor/.metadata/.plugins/org.eclipse.pde.core/Rodin.product/dev.properties -debug /home/htson/work/Rodin-Europa-Editor/.metadata/.plugins/org.eclipse.pde.core/Rodin.product/.options -os linux -ws gtk -arch x86

!ENTRY org.eventb.core.seqprover 4 4 2008-04-17 11:31:02.526
!MESSAGE Invalid id: org.eventb.core.seqprover.arithmetic (must not contain a dot)

!ENTRY org.eclipse.osgi 2 1 2008-04-17 11:34:07.577
!MESSAGE NLS unused message: symtab_cannotCreateAbstractConstant in: org.eventb.internal.core.sc.messages

!ENTRY org.eclipse.osgi 2 1 2008-04-17 11:34:07.578
!MESSAGE NLS unused message: symtab_cannotCreateAbstractCarrierSet in: org.eventb.internal.core.sc.messages

!ENTRY org.eclipse.ui 4 4 2008-04-17 13:49:58.537
!MESSAGE Save Failed

!ENTRY org.eclipse.ui 2 0 2008-04-17 13:49:58.569
!MESSAGE Save Failed
!STACK 0
java.lang.AssertionError
at org.eventb.internal.core.typecheck.TypeEnvironment.addName(TypeEnvironment.java:132)
at org.eventb.internal.core.typecheck.TypeEnvironment.add(TypeEnvironment.java:95)
at org.eventb.internal.core.typecheck.TypeEnvironment.addAll(TypeEnvironment.java:115)
at org.eventb.core.basis.ProofStoreReader.getPredicate(ProofStoreReader.java:74)
at org.eventb.core.basis.EventBProofElement.getHyps(EventBProofElement.java:184)
at org.eventb.core.basis.PRHypAction.getAction(PRHypAction.java:48)
at org.eventb.core.basis.PRRuleAntecedent.getAntecedent(PRRuleAntecedent.java:63)
at org.eventb.core.basis.PRProofRule.getProofSkeleton(PRProofRule.java:74)
at org.eventb.core.basis.EventBProofElement.getSkeleton(EventBProofElement.java:268)
at org.eventb.core.basis.PRProofRule.getProofSkeleton(PRProofRule.java:75)
at org.eventb.core.basis.EventBProofElement.getSkeleton(EventBProofElement.java:268)
at org.eventb.core.basis.PRProofRule.getProofSkeleton(PRProofRule.java:75)
at org.eventb.core.basis.EventBProofElement.getSkeleton(EventBProofElement.java:268)
at org.eventb.core.basis.PRProofRule.getProofSkeleton(PRProofRule.java:75)
at org.eventb.core.basis.EventBProofElement.getSkeleton(EventBProofElement.java:268)
at org.eventb.core.basis.PRProof.getSkeleton(PRProof.java:141)
at org.eventb.internal.core.PSWrapper.getProofSkeleton(PSWrapper.java:97)
at org.eventb.internal.core.pm.ProofState$1.run(ProofState.java:117)
at org.eventb.internal.core.pm.UserSupportManager.run(UserSupportManager.java:105)
at org.eventb.internal.core.pm.ProofState.loadProofTree(ProofState.java:96)
at org.eventb.internal.core.pm.UserSupport$8.run(UserSupport.java:506)
at org.eventb.internal.core.pm.UserSupportManager.run(UserSupportManager.java:105)
at org.eventb.internal.core.pm.UserSupport.elementChanged(UserSupport.java:493)
at org.rodinp.internal.core.DeltaProcessor$2.run(DeltaProcessor.java:636)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:37)
at org.rodinp.internal.core.DeltaProcessor.notifyListeners(DeltaProcessor.java:631)
at org.rodinp.internal.core.DeltaProcessor.firePostChangeDelta(DeltaProcessor.java:535)
at org.rodinp.internal.core.DeltaProcessor.fire(DeltaProcessor.java:512)
at org.rodinp.internal.core.RodinDBOperation.run(RodinDBOperation.java:737)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:1797)
at org.rodinp.core.RodinCore.run(RodinCore.java:552)
at org.rodinp.core.RodinCore.run(RodinCore.java:505)
at org.eventb.internal.ui.prover.ProverUI.doSave(ProverUI.java:306)
at org.eclipse.ui.internal.SaveableHelper$1.run(SaveableHelper.java:143)
at org.eclipse.ui.internal.SaveableHelper$4.run(SaveableHelper.java:266)
at org.eclipse.jface.operation.ModalContext.runInCurrentThread(ModalContext.java:369)
at org.eclipse.jface.operation.ModalContext.run(ModalContext.java:313)
at org.eclipse.jface.window.ApplicationWindow$1.run(ApplicationWindow.java:758)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:67)
at org.eclipse.jface.window.ApplicationWindow.run(ApplicationWindow.java:755)
at org.eclipse.ui.internal.WorkbenchWindow.run(WorkbenchWindow.java:2451)
at org.eclipse.ui.internal.SaveableHelper.runProgressMonitorOperation(SaveableHelper.java:274)
at org.eclipse.ui.internal.SaveableHelper.runProgressMonitorOperation(SaveableHelper.java:253)
at org.eclipse.ui.internal.SaveableHelper.savePart(SaveableHelper.java:148)
at org.eclipse.ui.internal.EditorManager.savePart(EditorManager.java:1345)
at org.eclipse.ui.internal.WorkbenchPage.savePart(WorkbenchPage.java:3184)
at org.eclipse.ui.internal.WorkbenchPage.saveEditor(WorkbenchPage.java:3197)
at org.eclipse.ui.internal.SaveAction.run(SaveAction.java:73)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:498)
at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:545)
at org.eclipse.jface.action.ActionContributionItem.access$2(ActionContributionItem.java:490)
at org.eclipse.jface.action.ActionContributionItem$6.handleEvent(ActionContributionItem.java:443)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:66)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1101)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3319)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:2971)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2389)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2353)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2219)
at org.eclipse.ui.internal.Workbench$4.run(Workbench.java:466)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:289)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:461)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:106)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:153)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:106)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:76)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:363)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:176)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:504)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:443)
at org.eclipse.equinox.launcher.Main.run(Main.java:1169)
at org.eclipse.equinox.launcher.Main.main(Main.java:1144)