public function dispatch(Expression $expr)
    {
        switch (true) {
            case $expr instanceof Comparison:
                return $this->walkComparison($expr);
            case $expr instanceof Value:
                return $this->walkValue($expr);
            case $expr instanceof CompositeExpression:
                return $this->walkCompositeExpression($expr);
            default:
                throw new RuntimeException('Unknown Expression ' . get_class($expr));
        }
    }