code: def _ces_distance_simple(C1, C2):
      
      # Make C1 refer to the bigger CES.
      if len(C2) > len(C1):
          C1, C2 = C2, C1
      destroyed = [c1 for c1 in C1 if not any(c1.emd_eq(c2) for c2 in C2)]
      return sum(c.phi * concept_distance(c, c.subsystem.null_concept)
                 for c in destroyed)
docs: Return the distance between two cause-effect structures.
  
      Assumes the only difference between them is that some concepts have
      disappeared.