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Linked ol layer, starting with an underscore to not be part of the proxy. *
This class is a used in the state of the application, which will be accessed behind a javascript proxy. This means that each modification made to its properties must come from outside, because they have to be made through the proxy, so that the modification can be listen. Therefore, this class must not contain any method which is updating a value directly For example, any method doing <this.xxx = value> is forbidden here, because the modification be known from the proxy
the name of the class to facilitate the identification of subclasses, even in Proxy objects.