code: def test_sub(self):
      pval = MockMemory.cur_ptr
      memptr = mock_alloc(8) + 4
  
      memptr2 = memptr - 4
      assert isinstance(memptr2, pinned_memory.PinnedMemoryPointer)
      assert pval == int(memptr2)
  
      memptr -= 4
      assert isinstance(memptr, pinned_memory.PinnedMemoryPointer)
      assert pval == int(memptr)
  
imports: import unittest
  from cupy.cuda import pinned_memory
label_desc: Use image node_id 3 for calling the TestMemoryPointer obj's underlying member method code with example usage: obj.test_sub() without return types