code: def testStructNestedSubstraction(self):
      s1 = schema.Struct(
          ("a", schema.Scalar()),
          (
              "b",
              schema.Struct(
                  ("c", schema.Scalar()),
                  ("d", schema.Scalar()),
                  ("e", schema.Scalar()),
                  ("f", schema.Scalar()),
              ),
          ),
      )
      s2 = schema.Struct(
          (
              "b",
              schema.Struct(
                  ("d", schema.Scalar()),
                  ("e", schema.Scalar()),
              ),
          ),
      )
      s = s1 - s2
      self.assertEqual(["a", "b:c", "b:f"], s.field_names())
  
imports: from caffe2.python import core, schema
  import numpy
  import unittest
  import pickle
  import random
label_desc: Use image node_id 17 for calling the TestDB obj's underlying member method code with example usage: obj.testStructNestedSubstraction() without return types