code: def test_reverse_lookup_from_nested_dict():
      d = {"a": {"b": 10, "c": 20}, "b": {"c": 5}, "z": {"c": 100}}
      reverse_dict = reverse_lookup_from_nested_dict(d)
      len(reverse_dict.keys()) == 2
      assert "c" in reverse_dict
      eq_(set(reverse_dict["c"]), {("a", 20), ("b", 5), ("z", 100)})
      assert "b" in reverse_dict
      eq_(reverse_dict["b"], [("a", 10)])
  
file_path: fancyimpute/test/test_dictionary_helpers.py
imports: import numpy
  from fancyimpute.dictionary_helpers import dense_matrix_from_pair_dictionary, dense_matrix_from_nested_dictionary, reverse_lookup_from_nested_dict, transpose_nested_dictionary
  from nose.tools import eq_
label_desc: Use image node_id 5 for calling a global function with example usage: test_reverse_lookup_from_nested_dict() without return types