docs: public static void
      collectPartitioningFilters(Collection<AbstractExpression> filterList,
                                 HashMap<AbstractExpression, Set<AbstractExpression> > equivalenceSet)
      {
          for (AbstractExpression expr : filterList) {
              if ( ! expr.isColumnEquivalenceFilter()) {
                  continue;
              }
              AbstractExpression leftExpr = expr.getLeft();
              AbstractExpression rightExpr = expr.getRight();
  
              // Any two asserted-equal expressions need to map to the same equivalence set,
              // which must contain them and must be the only such set that contains them.
              Set<AbstractExpression> eqSet1 = null;
              if (equivalenceSet.containsKey(leftExpr)) {
                  eqSet1 = equivalenceSet.get(leftExpr);
              }
              if (equivalenceSet.containsKey(rightExpr)) {
                  Set<AbstractExpression> eqSet2 = equivalenceSet.get(rightExpr);
                  if (eqSet1 == null) {
                      // Add new leftExpr into existing rightExpr's eqSet.
                      equivalenceSet.put(leftExpr, eqSet2);
                      eqSet2.add(leftExpr);
                  } else {
                      // Merge eqSets, re-mapping all the rightExpr's equivalents into leftExpr's eqset.
                      for (AbstractExpression eqMember : eqSet2) {
                          eqSet1.add(eqMember);
                          equivalenceSet.put(eqMember, eqSet1);
                      }
                  }
              } else {
                  if (eqSet1 == null) {
                      // Both leftExpr and rightExpr are new -- add leftExpr to the new eqSet first.
                      eqSet1 = new HashSet<AbstractExpression>();
                      equivalenceSet.put(leftExpr, eqSet1);
                      eqSet1.add(leftExpr);
                  }
                  // Add new rightExpr into leftExpr's eqSet.
                  equivalenceSet.put(rightExpr, eqSet1);
                  eqSet1.add(rightExpr);
              }
          }
      }