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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 child_isinstance(block, child_id, block_class_or_mixin):
 """ Efficiently check if a child of an XBlock is an instance of the given class. Arguments: block -- the parent (or ancestor) of the child block in question child_id -- the usage key of the child block we are wondering about block_class_or_mixin -- We return true if block's child indentified by child_id is an instance of this. This method is equivalent to isinstance(block.runtime.get_block(child_id), block_class_or_mixin) but is far more efficient, as it avoids the need to instantiate the child. """

    def_id = block.runtime.id_reader.get_definition_id(child_id)
    type_name = block.runtime.id_reader.get_block_type(def_id)
    child_class = block.runtime.load_block_type(type_name)
    return issubclass(child_class, block_class_or_mixin)