code: def test_flatten():
      assert flatten(Basic(S(1), S(2), Basic(S(3), S(4)))) == Basic(
          S(1), S(2), S(3), S(4)
      )
  
file_path: sympy/sympy/strategies/tests/test_rl.py
imports: from sympy.core.singleton import S
  from sympy.strategies.rl import rm_id, glom, flatten, unpack, sort, distribute, subs, rebuild
  from sympy.core.basic import Basic
  from sympy.core.add import Add
  from sympy.core.mul import Mul
  from sympy.core.symbol import symbols
  from sympy.abc import x
label_desc: Use image node_id 3 for calling a global function with example usage: test_flatten() without return types