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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def link(self, ref: Optional[Union[str, Element]], *, after_link: str = None, use_desc: bool=False, add_subset: bool=True) -> str: """ Create a link to ref if appropriate. @param ref: the name or value of a class, slot, type or the name of a built in type. @param after_link: Text to put between link and description @param use_desc: True means append a description after the link if available @param add_subset: True means add any subset information that is available @return: """

        obj = self.obj_for(ref) if isinstance(ref, str) else ref
        nl = '\n'
        if isinstance(obj, str) or obj is None or not self.is_secondary_ref(obj.name):
            return self.bbin(ref)
        if isinstance(obj, SlotDefinition):
            link_name = ((be(obj.domain) + '.') if obj.alias else '') + self.aliased_slot_name(obj)
            link_ref = underscore(obj.name)
        else:
            link_name = self.obj_name(obj)
            link_ref = link_name
        desc = self.desc_for(obj, use_desc)
        return f'[{link_name}]' \
               f'({link_ref}.{self.format})' + \
                 (f' *subsets*: ({"| ".join(obj.in_subset)})' if add_subset and obj.in_subset else '') + \
                 (f' {after_link} ' if after_link else '') + (f' - {desc.split(nl)[0]}' if desc else '')