diff --git a/src/Witness.cpp b/src/Witness.cpp
index 029f63a..f7d462b 100644
--- a/src/Witness.cpp
+++ b/src/Witness.cpp
@@ -3,50 +3,59 @@
 WitnessPlan::WitnessPlan (char const * const name, Plan * const input)
 	: Plan (name), _input (input)
 {
-	TRACE (TRACE_VAL);
-} // WitnessPlan::WitnessPlan
+	TRACE (true);
+} // WitnessPlan::WitnessPlan constructor
 
 WitnessPlan::~WitnessPlan ()
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 	delete _input;
-} // WitnessPlan::~WitnessPlan
+} // WitnessPlan::~WitnessPlan destructor
 
 Iterator * WitnessPlan::init () const
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 	return new WitnessIterator (this);
 } // WitnessPlan::init
 
 WitnessIterator::WitnessIterator (WitnessPlan const * const plan) :
 	_plan (plan), _input (plan->_input->init ()),
-	_rows (0)
+	_rows (0) , _parity(0) , _inversions(0), _hasPrevious(false) // initialise parity
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 } // WitnessIterator::WitnessIterator
 
 WitnessIterator::~WitnessIterator ()
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 
 	delete _input;
 
-	traceprintf ("%s witnessed %lu rows\n",
+	traceprintf ("%s witnessed %lu rows with parity %lu and %lu inversions\n",
 			_plan->_name,
-			(unsigned long) (_rows));
+			(unsigned long) (_rows),
+			(unsigned long) (_parity),
+			(unsigned long) (_inversions));
 } // WitnessIterator::~WitnessIterator
 
 bool WitnessIterator::next (Row & row)
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 
 	if ( ! _input->next (row))  return false;
 	++ _rows;
+	_parity ^= row.getId();  
+	// Compute XOR parity (assuming `row.id` is the attribute)
+	if (_hasPrevious && _previousRow.getId() > row.getId()) {
+        ++_inversions;
+    }
+	_previousRow = row;
+    _hasPrevious = true;
 	return true;
 } // WitnessIterator::next
 
 void WitnessIterator::free (Row & row)
 {
-	TRACE (TRACE_VAL);
+	TRACE (true);
 	_input->free (row);
 } // WitnessIterator::free