code: package it.unibz.inf.ontop.model.term.functionsymbol.impl;
  
  import com.google.common.collect.ImmutableList;
  import it.unibz.inf.ontop.iq.node.VariableNullability;
  import it.unibz.inf.ontop.model.term.ImmutableFunctionalTerm;
  import it.unibz.inf.ontop.model.term.ImmutableTerm;
  import it.unibz.inf.ontop.model.term.TermFactory;
  import it.unibz.inf.ontop.model.term.functionsymbol.BooleanFunctionSymbol;
  import it.unibz.inf.ontop.model.term.functionsymbol.FunctionSymbol;
  import it.unibz.inf.ontop.model.term.functionsymbol.RDFTermTypeFunctionSymbol;
  import it.unibz.inf.ontop.model.term.functionsymbol.db.DBIfThenFunctionSymbol;
  import it.unibz.inf.ontop.model.type.DBTermType;
  import it.unibz.inf.ontop.model.type.TermType;
  import it.unibz.inf.ontop.model.type.TermTypeInference;
  import it.unibz.inf.ontop.utils.ImmutableCollectors;
  
  import javax.annotation.Nonnull;
  import java.util.Optional;
  import java.util.stream.IntStream;
  
  public abstract class BooleanFunctionSymbolImpl extends FunctionSymbolImpl implements BooleanFunctionSymbol {
  
      private final DBTermType dbBooleanTermType;
  
      protected BooleanFunctionSymbolImpl(@Nonnull String name, @Nonnull ImmutableList<TermType> expectedBaseTypes,
                                          DBTermType dbBooleanTermType) {
          super(name, expectedBaseTypes);
          this.dbBooleanTermType = dbBooleanTermType;
      }
  
      @Override
      public Optional<TermTypeInference> inferType(ImmutableList<? extends ImmutableTerm> terms) {
          return Optional.of(TermTypeInference.declareTermType(dbBooleanTermType));
      }
  
      @Override
      protected ImmutableTerm buildTermAfterEvaluation(ImmutableList<ImmutableTerm> newTerms, TermFactory termFactory,
                                                       VariableNullability variableNullability) {
          return tryToLiftIfThenTerm(newTerms, termFactory, variableNullability)
                  .orElseGet(() -> super.buildTermAfterEvaluation(newTerms, termFactory, variableNullability));
      }
  
      protected Optional<ImmutableTerm> tryToLiftIfThenTerm(ImmutableList<ImmutableTerm> newTerms, TermFactory termFactory,
                                                            VariableNullability variableNullability) {
          Optional<ImmutableFunctionalTerm> firstIfThenTerm = newTerms.stream()
                  .filter(t -> t instanceof ImmutableFunctionalTerm)
                  .map(t -> (ImmutableFunctionalTerm) t)
                  .filter(t -> t.getFunctionSymbol() instanceof DBIfThenFunctionSymbol)
                  .findAny();
  
          return firstIfThenTerm
                  .map(f -> {
                      int index = newTerms.indexOf(f);
                      return ((DBIfThenFunctionSymbol) f.getFunctionSymbol()).pushDownExpression(
                              termFactory.getImmutableExpression(this, newTerms), index, termFactory)
                              .simplify(variableNullability);
                  });
      }
  
      protected Optional<ImmutableTerm> tryToLiftMagicNumbers(ImmutableList<ImmutableTerm> newTerms,
                                                            TermFactory termFactory,
                                                            VariableNullability variableNullability) {
          return tryToLiftMagicNumbers(newTerms, termFactory, variableNullability, true);
      }
  
      @Override
      protected boolean enableIfElseNullLifting() {
          return true;
      }
  
      @Override
      protected boolean enableCoalesceLifting() {
          return true;
      }
  }
  