 In this paper we aim at  clarifying the overall situation by proposing a clear classification of the methods and providing a comprehensive survey of the available results .
<clarity>  In this paper we aim at  clarifying the overall situation by proposing a clear classification of the methods and providing a comprehensive survey of the available results .
 In this paper we aim at describing and clarifying the overall situation by proposing a clear classification of the methods and providing a comprehensive survey of the available results .