code: def dimensions_from_subgroups(s):
      
      letters = 'XYABCDEFGHIJKLMNOPQRSTUVWZ'
      return ' '.join(['dim{0}={1}'.format(dim, sg.name) for dim, sg in zip(letters, s)])
docs: Given a sorted list of subgroups, return a string appropriate to provide as
      a composite track's `dimensions` arg.
  
      Parameters
      ----------
      s : list of SubGroup objects (or anything with a `name` attribute)