protected void ensureSimpleValues(Map.Entry<String,Set<RDFNode>> prop, Class<?> expectedClass)
		throws RDFStructureException
	{
		for( RDFNode n : prop.getValue() ) {
			if( !n.hasOnlySimpleValue() ) {
				throw new RDFStructureException("Expected a simple value for "+prop.getKey(), n.sourceUri);
			}
			if( n.simpleValue != null && !expectedClass.isInstance(n.simpleValue) ) {
				throw new RDFStructureException("Expected "+prop.getKey()+" to be a "+expectedClass.getSimpleName()+
					", but found a "+n.simpleValue.getClass().getSimpleName(), n.sourceUri);
			}
		}
	}