  Let U be an open set in R^d. We show that under a mild assumption on the
richness of the generator a Feller process in U with (predictable) killing is a
semimartingale. To this end we generalize the notion of semimartingales in a
natural way to those 'with killing'. Furthermore we calculate the
semimartingale characteristics of the Feller process explicitly and analyze
their connections to the symbol. Finally we derive a probabilistic formula to
calculate the symbol of the process directly.
