code: package it.unibz.inf.ontop.model.term.functionsymbol.db.impl;
  
  import com.google.common.collect.ImmutableList;
  import com.google.common.collect.ImmutableMap;
  import com.google.common.collect.ImmutableSortedSet;
  import com.google.common.collect.Maps;
  import it.unibz.inf.ontop.exception.MinorOntopInternalBugException;
  import it.unibz.inf.ontop.iq.node.VariableNullability;
  import it.unibz.inf.ontop.model.term.ImmutableExpression;
  import it.unibz.inf.ontop.model.term.ImmutableTerm;
  import it.unibz.inf.ontop.model.term.NonFunctionalTerm;
  import it.unibz.inf.ontop.model.term.TermFactory;
  import it.unibz.inf.ontop.model.term.functionsymbol.db.DBIsNullOrNotFunctionSymbol;
  import it.unibz.inf.ontop.model.type.DBTermType;
  import it.unibz.inf.ontop.utils.ImmutableCollectors;
  
  import java.util.Map;
  import java.util.Optional;
  import java.util.stream.IntStream;
  
  /**
   * For AND, OR, etc.
   */
  public abstract class AbstractDBBooleanConnectorFunctionSymbol extends DBBooleanFunctionSymbolImpl {
  
      protected AbstractDBBooleanConnectorFunctionSymbol(String name, int arity, DBTermType dbBooleanTermType) {
          super(name + arity,
                  IntStream.range(0, arity)
                          .mapToObj(i -> dbBooleanTermType)
                          .collect(ImmutableCollectors.toList()),
                  dbBooleanTermType);
      }
  
      @Override
      protected boolean tolerateNulls() {
          return true;
      }
  
      /**
       * Propagates the info that 2VL can be applied to the sub-expressions.
       */
      @Override
      public ImmutableTerm simplify2VL(ImmutableList<? extends ImmutableTerm> terms, TermFactory termFactory,
                                       VariableNullability variableNullability) {
          ImmutableList<ImmutableTerm> newSubTerms = terms.stream()
                  .map(t -> (t instanceof ImmutableExpression)
                          ? ((ImmutableExpression) t).simplify2VL(variableNullability)
                          : t)
                  .collect(ImmutableCollectors.toList());
  
          ImmutableList<ImmutableTerm> termsAfterInteraction = simplify2VLInteractions(newSubTerms, termFactory,
                  variableNullability);
  
          return buildTermAfterEvaluation(termsAfterInteraction, termFactory, variableNullability);
      }
  
      protected abstract ImmutableList<ImmutableTerm> simplify2VLInteractions(
              ImmutableList<ImmutableTerm> newTerms, TermFactory termFactory, VariableNullability variableNullability);
  
      /**
       * Requires its arguments to be expressions
       */
      @Override
      protected ImmutableList<? extends ImmutableTerm> transformIntoRegularArguments(
              ImmutableList<? extends NonFunctionalTerm> arguments, TermFactory termFactory) {
          return arguments.stream()
                  .map(termFactory::getIsTrue)
                  .collect(ImmutableCollectors.toList());
      }
  
  
      /**
       * Look for conjuncts that are IS_NULL(...) or disjuncts that are IS_NOT_NULL(...)
       * and uses them to nullify some terms
       */
      protected ImmutableList<ImmutableTerm> simplifyIsNullOrIsNotNull(ImmutableList<ImmutableTerm> newTerms, TermFactory termFactory,
                                                                       VariableNullability variableNullability,
                                                                       boolean lookForIsNull) {
  
          // Removes duplicates to avoid returning false in cases like OR2(IS_NOT_NULL(b),IS_NOT_NULL(b))
          ImmutableList<ImmutableTerm> distinctTerms = newTerms.stream()
                  .distinct()
                  .collect(ImmutableCollectors.toList());
  
          // { index -> termToNullify }
          ImmutableMap<Integer, ImmutableTerm> termToNullifyMap = IntStream.range(0, distinctTerms.size())
                  .mapToObj(i -> Maps.immutableEntry(i, Optional.of(distinctTerms.get(i))
                          .filter(t -> t instanceof ImmutableExpression)
                          .map(t -> (ImmutableExpression) t)
                          .filter(e -> (e.getFunctionSymbol() instanceof DBIsNullOrNotFunctionSymbol)
                                  && (((DBIsNullOrNotFunctionSymbol) e.getFunctionSymbol()).isTrueWhenNull() == lookForIsNull))
                          .map(e -> e.getTerm(0))))
                  .filter(e -> e.getValue().isPresent())
                  .collect(ImmutableCollectors.toMap(
                          Map.Entry::getKey,
                          e -> e.getValue().get()));
  
          return termToNullifyMap.entrySet().stream()
                  .reduce(distinctTerms,
                          (ts, e) -> IntStream.range(0, distinctTerms.size())
                                  .mapToObj(i -> e.getKey().equals(i)
                                          ? ts.get(i)
                                          // Only tries to nullify other entries.
                                          // NB: nullifying only replaces the same syntactic term by NULL
                                          : Nullifiers.nullify(ts.get(i), e.getValue(), termFactory)
                                              .simplify(variableNullability))
                                  .collect(ImmutableCollectors.toList()),
                          (ts1, ts2) -> {
                              throw new MinorOntopInternalBugException("No merging was expected");
                          });
      }
  }
  