code: def bel_to_spia_matrices(graph: BELGraph) -> Mapping[str, pd.DataFrame]:
      
      index_nodes = get_matrix_index(graph)
      spia_matrices = build_spia_matrices(index_nodes)
  
      for u, v, edge_data in graph.edges(data=True):
          # Both nodes are CentralDogma abundances
          if isinstance(u, CentralDogma) and isinstance(v, CentralDogma):
              # Update matrix dict
              update_spia_matrices(spia_matrices, u, v, edge_data)
  
          # Subject is CentralDogmaAbundance and node is ListAbundance
          elif isinstance(u, CentralDogma) and isinstance(v, ListAbundance):
              # Add a relationship from subject to each of the members in the object
              for node in v.members:
                  # Skip if the member is not in CentralDogma
                  if not isinstance(node, CentralDogma):
                      continue
  
                  update_spia_matrices(spia_matrices, u, node, edge_data)
  
          # Subject is ListAbundance and node is CentralDogmaAbundance
          elif isinstance(u, ListAbundance) and isinstance(v, CentralDogma):
              # Add a relationship from each of the members of the subject to the object
              for node in u.members:
                  # Skip if the member is not in CentralDogma
                  if not isinstance(node, CentralDogma):
                      continue
  
                  update_spia_matrices(spia_matrices, node, v, edge_data)
  
          # Both nodes are ListAbundance
          elif isinstance(u, ListAbundance) and isinstance(v, ListAbundance):
              for sub_member, obj_member in product(u.members, v.members):
                  # Update matrix if both are CentralDogma
                  if isinstance(sub_member, CentralDogma) and isinstance(obj_member, CentralDogma):
                      update_spia_matrices(spia_matrices, sub_member, obj_member, edge_data)
  
          # else Not valid edge
  
      return spia_matrices
docs: Create an excel sheet ready to be used in SPIA software.
  
      :param graph: BELGraph
      :return: dictionary with matrices