Assert.isTrue(child instanceof PartPlaceholder
|| child instanceof PartPane,
"PartStack can only contain PartPlaceholders or PartPanes"); //$NON-NLS-1$
// Ensure that all the PartPanes have an associated presentable part
IPresentablePart part = getPresentablePart(child);
if (child instanceof PartPane) {
Assert.isNotNull(part,
"All PartPanes must have a non-null IPresentablePart"); //$NON-NLS-1$
}