@abstractmethod
    def num_tuples(self):
Instruct all children to perform dereferencing.
dataset/ETHPy150Open uwescience/raco/raco/algebra.py/Operator.num_tuples--crossover with--dataset/ETHPy150Open jumoconnect/openjumo/jumodjango/lib/suds/xsd/schema.py/Schema.dereference