code: def _merge_simple_selectors(a, b):
      
      # TODO what about combinators
      if a.is_superset_of(b):
          return b
      elif b.is_superset_of(a):
          return a
      else:
          return None
docs: Merge two simple selectors, for the purposes of the LCS algorithm below.
  
      In practice this returns the more specific selector if one is a subset of
      the other, else it returns None.