protected Instance mergeInstances(Instance source, Instance dest) {
Instances outputFormat = outputFormatPeek();
double[] vals = new double[outputFormat.numAttributes()];
for(int i = 0; i < vals.length; i++) {
if ((i != outputFormat.classIndex()) && (m_SelectedCols.isInRange(i))) {
if ((source != null) && !source.isMissing(i) && !dest.isMissing(i)) {
vals[i] = dest.value(i) - source.value(i);
} else {
vals[i] = Instance.missingValue();
}