final PropertyListUnparsed propertyListUnparsed
= (PropertyListUnparsed) propertyList;
if (parent instanceof InstreamForeignObject) {
final InstreamForeignObject ifo =
(InstreamForeignObject) parent;
return new InstreamMathElement(ifo, propertyListUnparsed);
} else {
parent.throwException("Instream MathML content must be inside "
+ "an fo:instream-foreign-object.");
return null;
}