code: def get_vert_opposites_per_edge(mesh_v, mesh_f):
      
      result = {}
      for f in mesh_f:
          for i in range(3):
              key = [f[i], f[(i+1)%3]]
              key.sort()
              key = tuple(key)
              val = f[(i+2)%3]
  
              if key in result:
                  result[key].append(val)
              else:
                  result[key] = [val]
      return result
docs: Returns a dictionary from vertidx-pairs to opposites.
      For example, a key consist of [4,5)] meaning the edge between
      vertices 4 and 5, and a value might be [10,11] which are the indices
      of the vertices opposing this edge.