private void convertNotEquals(E_NotEquals expr) {
    logger.debug("convertNotEquals " + expr.toString());

    expr.getArg1().visit(this);
    expr.getArg2().visit(this);
    Expression e2 = expression.pop();
    Expression e1 = expression.pop();

    // TODO Expression.FALSE and Expression.TRUE are not constants
    if (e1.equals(Expression.FALSE)) e1 = CONSTANT_FALSE;
    else if (e1.equals(Expression.TRUE)) e1 = CONSTANT_TRUE;
    if (e2.equals(Expression.FALSE)) e2 = CONSTANT_FALSE;
    else if (e2.equals(Expression.TRUE)) e2 = CONSTANT_TRUE;

    if (e1 instanceof AttributeExprEx && e2 instanceof Constant
        || e2 instanceof AttributeExprEx && e1 instanceof Constant) {
      AttributeExprEx variable;
      ConstantEx constant;

      if (e1 instanceof AttributeExprEx) {
        variable = (AttributeExprEx) e1;
        constant = (ConstantEx) e2;
      } else {
        variable = (AttributeExprEx) e2;
        constant = (ConstantEx) e1;
      }

      logger.debug("isNotEqual(" + variable + ", " + constant + ")");

      NodeMaker nm = variable.getNodeMaker();

      if (nm instanceof TypedNodeMaker) {
        ValueMaker vm = ((TypedNodeMaker) nm).valueMaker();
        Node node = constant.getNode();
        logger.debug("checking " + node + " with " + nm);
        if (XSD.isNumeric(node)) {
          DetermineNodeType filter = new DetermineNodeType();
          nm.describeSelf(filter);
          RDFDatatype datatype = filter.getDatatype();
          if (datatype != null && XSD.isNumeric(datatype)) {
            RDFDatatype numericType = XSD.getNumericType(datatype, node.getLiteralDatatype());
            nm = cast(nm, numericType);
            node = XSD.cast(node, numericType);
          }
        }
        boolean empty = nm.selectNode(node, RelationalOperators.DUMMY).equals(NodeMaker.EMPTY);
        logger.debug("result " + new Boolean(empty));
        if (!empty) {
          if (node.isURI()) expression.push(new Negation(vm.valueExpression(node.getURI())));
          else if (node.isLiteral()) {
            if (XSD.isSupported(node.getLiteralDatatype()))
              expression.push(new Negation(vm.valueExpression(constant.value())));
            else // type = boolean or an unknown type
            conversionFailed("cannot compare values of type " + node.getLiteralDatatypeURI(), expr);
          } else conversionFailed(expr); // TODO blank nodes?
          return;
        } else {
          expression.push(Expression.TRUE);
          return;
        }
      }
    } else if (e1 instanceof ConstantEx && e2 instanceof ConstantEx) {
      logger.debug("isNotEqual(" + e1 + ", " + e2 + ")");
      Node c1 = ((ConstantEx) e1).getNode();
      Node c2 = ((ConstantEx) e2).getNode();
      boolean equals;
      if (XSD.isNumeric(c1) && XSD.isNumeric(c2)) {
        RDFDatatype datatype = XSD.getNumericType(c1.getLiteralDatatype(), c2.getLiteralDatatype());
        equals = XSD.cast(c1, datatype).equals(XSD.cast(c2, datatype));
      } else if (isSimpleLiteral(c1) && isSimpleLiteral(c2)) {
        equals = c1.getLiteralValue().equals(c2.getLiteralValue());
      } else if (XSD.isString(c1) && XSD.isString(c2)) {
        equals = c1.getLiteralValue().equals(c2.getLiteralValue());
      } else {
        try {
          equals = NodeFunctions.rdfTermEquals(c1, c2);
        } catch (ExprEvalException e) {
          equals = false;
        }
      }
      logger.debug("constants equal? " + new Boolean(equals));
      expression.push(equals ? Expression.FALSE : Expression.TRUE);
      return;
    } else if (e1 instanceof AttributeExprEx && e2 instanceof AttributeExprEx) {
      logger.debug("isNotEqual(" + e1 + ", " + e2 + ")");
      AttributeExprEx variable1 = (AttributeExprEx) e1;
      AttributeExprEx variable2 = (AttributeExprEx) e2;

      NodeMaker nm1 = variable1.getNodeMaker();
      NodeMaker nm2 = variable2.getNodeMaker();

      DetermineNodeType filter1 = new DetermineNodeType();
      nm1.describeSelf(filter1);
      RDFDatatype datatype1 = filter1.getDatatype();

      DetermineNodeType filter2 = new DetermineNodeType();
      nm2.describeSelf(filter2);
      RDFDatatype datatype2 = filter2.getDatatype();

      if (datatype1 != null
          && XSD.isNumeric(datatype1)
          && datatype2 != null
          && XSD.isNumeric(datatype2)) {
        RDFDatatype numericType = XSD.getNumericType(filter1.getDatatype(), filter2.getDatatype());
        nm1 = cast(nm1, numericType);
        nm2 = cast(nm2, numericType);
      }

      NodeSetConstraintBuilder nodeSet = new NodeSetConstraintBuilder();
      nm1.describeSelf(nodeSet);
      nm2.describeSelf(nodeSet);

      if (nodeSet.isEmpty()) {
        logger.debug("nodes " + nm1 + " " + nm2 + " incompatible");
        expression.push(Expression.TRUE);
        return;
      }
    }

    expression.push(new Negation(Equality.create(e1, e2)));
  }
  /*
   * See http://www.w3.org/TR/rdf-sparql-query paragraph 11.4.11
   *
   * "(SAMETERM) Returns TRUE if term1 and term2 are the same RDF term as defined
   * in Resource Description Framework (RDF): Concepts and Abstract Syntax [CONCEPTS]; returns FALSE otherwise."
   *
   * @see http://www.w3.org/TR/rdf-sparql-query
   * @see http://www.w3.org/TR/rdf-concepts/
   */
  private void convertSameTerm(E_SameTerm expr) {
    logger.debug("convertSameTerm " + expr.toString());

    expr.getArg1().visit(this);
    expr.getArg2().visit(this);
    Expression e2 = expression.pop();
    Expression e1 = expression.pop();

    // TODO Expression.FALSE and Expression.TRUE are not constants
    if (e1.equals(Expression.FALSE)) e1 = CONSTANT_FALSE;
    else if (e1.equals(Expression.TRUE)) e1 = CONSTANT_TRUE;
    if (e2.equals(Expression.FALSE)) e2 = CONSTANT_FALSE;
    else if (e2.equals(Expression.TRUE)) e2 = CONSTANT_TRUE;

    if (e1 instanceof AttributeExprEx && e2 instanceof Constant
        || e2 instanceof AttributeExprEx && e1 instanceof Constant) {

      AttributeExprEx variable;
      ConstantEx constant;

      if (e1 instanceof AttributeExprEx) {
        variable = (AttributeExprEx) e1;
        constant = (ConstantEx) e2;
      } else {
        variable = (AttributeExprEx) e2;
        constant = (ConstantEx) e1;
      }

      logger.debug("isEqual(" + variable + ", " + constant + ")");

      NodeMaker nm = variable.getNodeMaker();

      if (nm instanceof TypedNodeMaker) {
        ValueMaker vm = ((TypedNodeMaker) nm).valueMaker();
        Node node = constant.getNode();
        logger.debug("checking " + node + " with " + nm);

        boolean empty = nm.selectNode(node, RelationalOperators.DUMMY).equals(NodeMaker.EMPTY);
        logger.debug("result " + new Boolean(empty));
        if (!empty) {
          if (node.isURI()) expression.push(vm.valueExpression(node.getURI()));
          else if (node.isLiteral()) expression.push(vm.valueExpression(constant.value()));
          else conversionFailed(expr); // TODO blank nodes?
          return;
        } else {
          expression.push(Expression.FALSE);
          return;
        }
      } else {
        logger.warn("nm is not a TypedNodemaker");
      }
    } else if (e1 instanceof ConstantEx && e2 instanceof ConstantEx) {
      logger.debug("isEqual(" + e1 + ", " + e2 + ")");
      ConstantEx constant1 = (ConstantEx) e1;
      ConstantEx constant2 = (ConstantEx) e2;
      boolean equals = NodeFunctions.sameTerm(constant1.getNode(), constant2.getNode());
      logger.debug("constants same? " + new Boolean(equals));
      expression.push(equals ? Expression.TRUE : Expression.FALSE);
      return;
    } else if (e1 instanceof AttributeExprEx && e2 instanceof AttributeExprEx) {
      logger.debug("isEqual(" + e1 + ", " + e2 + ")");
      AttributeExprEx variable1 = (AttributeExprEx) e1;
      AttributeExprEx variable2 = (AttributeExprEx) e2;

      NodeMaker nm1 = variable1.getNodeMaker();
      NodeMaker nm2 = variable2.getNodeMaker();

      NodeSetConstraintBuilder nodeSet = new NodeSetConstraintBuilder();
      nm1.describeSelf(nodeSet);
      nm2.describeSelf(nodeSet);

      if (nodeSet.isEmpty()) {
        logger.debug("nodes " + nm1 + " " + nm2 + " incompatible");
        expression.push(Expression.FALSE);
        return;
      }
    }

    expression.push(Equality.create(e1, e2));
  }
  private void convertFunction(ExprFunction2 expr) {
    logger.debug("convertFunction " + expr.toString());

    if (expr instanceof E_LogicalOr) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(e1.or(e2));
    } else if (expr instanceof E_LessThan) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new LessThan(e1, e2));
    } else if (expr instanceof E_LessThanOrEqual) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new LessThanOrEqual(e1, e2));
    } else if (expr instanceof E_GreaterThan) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new GreaterThan(e1, e2));
    } else if (expr instanceof E_GreaterThanOrEqual) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new GreaterThanOrEqual(e1, e2));
    } else if (expr instanceof E_Add) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new Add(e1, e2));
    } else if (expr instanceof E_Subtract) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new Subtract(e1, e2));
    } else if (expr instanceof E_Multiply) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new Multiply(e1, e2));
    } else if (expr instanceof E_Divide) {
      expr.getArg1().visit(this);
      expr.getArg2().visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();
      expression.push(new Divide(e1, e2));
    } else if (expr instanceof E_Equals) {
      convertEquals((E_Equals) expr);
    } else if (expr instanceof E_NotEquals) {
      convertNotEquals((E_NotEquals) expr);
    } else if (expr instanceof E_LangMatches) {
      convertLangMatches((E_LangMatches) expr);
    } else if (expr instanceof E_SameTerm) {
      convertSameTerm((E_SameTerm) expr);
    } else if (extensionSupports(expr)) {
      expr.getArg(1).visit(this);
      expr.getArg(2).visit(this);
      Expression e2 = expression.pop();
      Expression e1 = expression.pop();

      List<Expression> args = new ArrayList<Expression>(2);

      args.add(e1);
      args.add(e2);

      extensionConvert(expr, args);
    } else {
      conversionFailed(expr);
    }
  }