As Taverna can't (currently) handle null (see http://www.mygrid.org.uk/dev/issues/browse/TAV-653) - in Taverna 2 one can instead return exceptions on individual ports and inside lists. Exceptions are registered as error documents by Taverna, and are passed along. The ErrorBounce layer of processors downstream will prevent execution if they see this 'null'-exception - as showed in this example that the string concatination is just run for the two list elements that are not exceptio...