code: def tearDown(self):
      del self.reader_file
      del self.reader_string
  
imports: import io
  import sys
  import unittest
  import xml.etree.ElementTree
  from pgmpy.readwrite import PomdpXReader, PomdpXWriter
label_desc: Use image node_id 12 for calling the TestPomdpXReaderString obj's underlying member method code with example usage: obj.tearDown() without return types