code: public AbstractJobVertex areVertexDegreesCorrect() {
  
  		// Check input vertices
  		final Iterator<AbstractJobInputVertex> iter = getInputVertices();
  		while (iter.hasNext()) {
  
  			final AbstractJobVertex jv = iter.next();
  
  			if (jv.getNumberOfForwardConnections() < 1 || jv.getNumberOfBackwardConnections() > 0) {
  				return jv;
  			}
  		}
  
  		// Check task vertices
  		final Iterator<JobTaskVertex> iter2 = getTaskVertices();
  		while (iter2.hasNext()) {
  
  			final AbstractJobVertex jv = iter2.next();
  
  			if (jv.getNumberOfForwardConnections() < 1 || jv.getNumberOfBackwardConnections() < 1) {
  				return jv;
  			}
  		}
  
  		// Check output vertices
  		final Iterator<AbstractJobOutputVertex> iter3 = getOutputVertices();
  		while (iter3.hasNext()) {
  
  			final AbstractJobVertex jv = iter3.next();
  
  			if (jv.getNumberOfForwardConnections() > 0 || jv.getNumberOfBackwardConnections() < 1) {
  				return jv;
  			}
  		}
  
  		return null;
  	}
docs: Checks for all registered job vertices if their in-/out-degree is correct.
  
  @return <code>null</code> if the in-/out-degree of all vertices is correct or the first job vertex whose
  in-/out-degree is incorrect.