String oldplacement = (String) parent.getChildConstraints(child);
if (!placement.equals(oldplacement)) {
IOperationHistory operationHistory = PlatformUI.getWorkbench()
.getOperationSupport().getOperationHistory();
IUndoableOperation operation = new JSplitPanePlacementOperation(container, child, placement);
operation.addContext(parent.getUndoContext());
try {
operationHistory.execute(operation, null, null);
} catch (ExecutionException e) {
WidgetPlugin.getLogger().error(e);
}