/* @see com.hp.hpl.jena.sparql.syntax.ElementVisitorBase#visit(com.hp.hpl.jena.sparql.syntax.ElementFilter) */
    @Override
    public void visit(final ElementFilter elementfilter) {
      final Expr expr = elementfilter.getExpr();
      final BuiltinAtom atom;
      try {
        if (expr instanceof Expression) // RDQL
        {
          final Expression e = (Expression) expr;
          final SWRLDataObject arg1 = makeDataObject(e.getArg(0));
          final SWRLDataObject arg2 = makeDataObject(e.getArg(1));

          if (e instanceof Q_Equal) atom = swrlFactory.createEqual(arg1, arg2);
          else if (e instanceof Q_NotEqual) atom = swrlFactory.createNotEqual(arg1, arg2);
          else if (e instanceof Q_GreaterThan) atom = swrlFactory.createGreaterThan(arg1, arg2);
          else if (e instanceof Q_GreaterThanOrEqual)
            atom = swrlFactory.createGreaterThanOrEqual(arg1, arg2);
          else if (e instanceof Q_LessThan) atom = swrlFactory.createLessThan(arg1, arg2);
          else if (e instanceof Q_LessThanOrEqual)
            atom = swrlFactory.createLessThanOrEqual(arg1, arg2);
          else
            throw new IllegalArgumentException(
                "Unsupported constraint expression " + e + " used in RDQL query.");
        } else if (expr.isFunction()) // SPARQL
        {
          final ExprFunction f = (ExprFunction) expr;
          final SWRLDataObject arg1 = makeDataObject(f.getArg(0));
          final SWRLDataObject arg2 = makeDataObject(f.getArg(1));

          if (f instanceof E_Equals) atom = swrlFactory.createEqual(arg1, arg2);
          else if (f instanceof E_NotEquals) atom = swrlFactory.createNotEqual(arg1, arg2);
          else if (f instanceof E_GreaterThan) atom = swrlFactory.createGreaterThan(arg1, arg2);
          else if (f instanceof E_GreaterThanOrEqual)
            atom = swrlFactory.createGreaterThanOrEqual(arg1, arg2);
          else if (f instanceof E_LessThan) atom = swrlFactory.createLessThan(arg1, arg2);
          else if (f instanceof E_LessThanOrEqual)
            atom = swrlFactory.createLessThanOrEqual(arg1, arg2);
          else
            throw new IllegalArgumentException(
                "Unsupported constraint (filter) " + f + " used in SPARQL query.");
        } else return;
        atoms = atoms.cons(atom);
      } catch (final URISyntaxException e) {
        throw new IllegalArgumentException(e.getInput() + " is not a valid URI!");
      }
    }