def reduce_length(key, values):
  yield str((key, len(values)))
build prerequisites table from rules
dataset/ETHPy150Open GoogleCloudPlatform/appengine-mapreduce/python/test/mapreduce/large_mapreduce_test.py/reduce_length--crossover with--dataset/ETHPy150Open sympy/sympy/sympy/core/facts.py/rules_2prereq