code: def test_functions_of():
      def a(x):
          return x
  
      def b(x):
          return x
  
      assert functions_of((a, 1)) == {a}
      assert functions_of((a, (b, 1))) == {a, b}
      assert functions_of((a, [(b, 1)])) == {a, b}
      assert functions_of((a, [[[(b, 1)]]])) == {a, b}
      assert functions_of(1) == set()
      assert functions_of(a) == set()
      assert functions_of((a,)) == {a}
  
imports: import itertools
  import pickle
  from functools import partial
  import pytest
  from flash.core.serve.dag.optimization import SubgraphCallable, cull, functions_of, fuse, fuse_linear, inline, inline_functions
  from flash.core.serve.dag.task import get, get_dependencies
  from flash.core.serve.dag.utils import apply, partial_by_order
  from flash.core.serve.dag.utils_test import add, inc
  from flash.core.utilities.imports import _TOPIC_SERVE_AVAILABLE
label_desc: Use image node_id 14 for calling a global function with example usage: test_functions_of() and returns: x, x