public ValueAndModified computeValueAndModified(ValueTuple vt) {
int source_mod = base.getModified(vt);
if (source_mod == ValueTuple.MISSING_NONSENSICAL) return ValueAndModified.MISSING_NONSENSICAL;
if (source_mod == ValueTuple.MISSING_FLOW) return ValueAndModified.MISSING_FLOW;
return computeValueAndModifiedImpl(vt);
}
