code: def build_compressed_graph(self):
      compressed_matrix = [
          ("1", ["B", "C"]),
          ("2", ["ABC"]),
          ("3", ["6AB"]),
          ("4", ["ABC"]),
          ("5", ["6AB"]),
          ("6", ["6AB", "A"]),
          ("A", ["6AB", "ABC"]),
          ("B", ["ABC", "6AB"]),
          ("C", ["ABC"]),
      ]
      compressed_graph = nx.Graph()
      for source, targets in compressed_matrix:
          for target in targets:
              compressed_graph.add_edge(source, target)
      return compressed_graph
  
imports: import pytest
  import networkx
label_desc: Use image node_id 4 for calling the TestUnDirectedDedensification obj's underlying member method code with example usage: obj.build_compressed_graph() and returns: compressed_graph