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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 get_possible_types( self, abstract_type: GraphQLAbstractType ) -> Sequence[GraphQLObjectType]: """Get list of all possible concrete types for given abstract type."""

        if is_union_type(abstract_type):
            abstract_type = cast(GraphQLUnionType, abstract_type)
            return abstract_type.types
        return self._implementations[abstract_type.name]