code: def test_state_transition_function_dd(self):
      self.model_data = {
          "state_transition_function": [
              {
                  "Var": "rover_1",
                  "Parent": ["action_rover", "rover_0"],
                  "Type": "DD",
                  "Parameter": {
                      "action_rover": {
                          "amw": {
                              "rover_0": {
                                  "s0": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                                  "s1": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s0",
                                  },
                                  "s2": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                              }
                          },
                          "ame": {
                              "rover_0": {
                                  "s0": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s1",
                                  },
                                  "s1": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                                  "s2": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                              }
                          },
                          "ac": {
                              "type": "persistent",
                              "var": "rover_1",
                          },
                          "as": {
                              "rover_0": {
                                  "s0": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s0",
                                  },
                                  "s1": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                                  "s2": {
                                      "type": "deterministic",
                                      "var": "rover_1",
                                      "val": "s2",
                                  },
                              }
                          },
                      }
                  },
              },
              {
                  "Var": "rock_1",
                  "Parent": ["action_rover", "rover_0", "rock_0"],
                  "Type": "DD",
                  "Parameter": {
                      "action_rover": {
                          "amw": {
                              "type": "persistent",
                              "var": "rock_1",
                          },
                          "ame": {
                              "type": "persistent",
                              "var": "rock_1",
                          },
                          "ac": {"type": "persistent", "var": "rock_1"},
                          "as": {
                              "rover_0": {
                                  "s0": {
                                      "type": "deterministic",
                                      "var": "rock_1",
                                      "val": "bad",
                                  },
                                  "s1": {
                                      "type": "persistent",
                                      "var": "rock_1",
                                  },
                                  "s2": {
                                      "type": "persistent",
                                      "var": "rock_1",
                                  },
                              }
                          },
                      }
                  },
              },
          ]
      }
  
      self.writer = PomdpXWriter(model_data=self.model_data)
      expected_xml = etree.XML(
          """
  <StateTransitionFunction>
    <CondProb>
      <Var>rover_1</Var>
      <Parent>action_rover rover_0</Parent>
      <Parameter type="DD">
        <DAG>
          <Node var="action_rover">
            <Edge val="ac">
              <SubDAG type="persistent" var="rover_1"/>
            </Edge>
            <Edge val="ame">
              <Node var="rover_0">
                <Edge val="s0">
                  <SubDAG type="deterministic" val="s1" var="rover_1"/>
                </Edge>
                <Edge val="s1">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
                <Edge val="s2">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
              </Node>
            </Edge>
            <Edge val="amw">
              <Node var="rover_0">
                <Edge val="s0">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
                <Edge val="s1">
                  <SubDAG type="deterministic" val="s0" var="rover_1"/>
                </Edge>
                <Edge val="s2">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
              </Node>
            </Edge>
            <Edge val="as">
              <Node var="rover_0">
                <Edge val="s0">
                  <SubDAG type="deterministic" val="s0" var="rover_1"/>
                </Edge>
                <Edge val="s1">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
                <Edge val="s2">
                  <SubDAG type="deterministic" val="s2" var="rover_1"/>
                </Edge>
              </Node>
            </Edge>
          </Node>
        </DAG>
      </Parameter>
    </CondProb>
    <CondProb>
      <Var>rock_1</Var>
      <Parent>action_rover rover_0 rock_0</Parent>
      <Parameter type="DD">
        <DAG>
          <Node var="action_rover">
            <Edge val="ac">
              <SubDAG type="persistent" var="rock_1"/>
            </Edge>
            <Edge val="ame">
              <SubDAG type="persistent" var="rock_1"/>
            </Edge>
            <Edge val="amw">
              <SubDAG type="persistent" var="rock_1"/>
            </Edge>
            <Edge val="as">
              <Node var="rover_0">
                <Edge val="s0">
                  <SubDAG type="deterministic" val="bad" var="rock_1"/>
                </Edge>
                <Edge val="s1">
                  <SubDAG type="persistent" var="rock_1"/>
                </Edge>
                <Edge val="s2">
                  <SubDAG type="persistent" var="rock_1"/>
                </Edge>
              </Node>
            </Edge>
          </Node>
        </DAG>
      </Parameter>
    </CondProb>
  </StateTransitionFunction>"""
      )
      self.maxDiff = None
      self.assertEqual(
          str(self.writer.add_state_transition_function()),
          str(etree.tostring(expected_xml)),
      )
  
imports: import io
  import sys
  import unittest
  import xml.etree.ElementTree
  from pgmpy.readwrite import PomdpXReader, PomdpXWriter
label_desc: Use image node_id 8 for calling the TestPomdpXWriter obj's underlying member method code with example usage: obj.test_state_transition_function_dd() without return types