<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def union(self, *others, **kwargs):
 """ Returns the union of variants in a several VariantCollection objects. """

        return self._combine_variant_collections(
            combine_fn=set.union,
            variant_collections=(self,) + others,
            kwargs=kwargs)