code: def filter_composite_from_subgroups(s):
      
      dims = []
      for letter, sg in zip('ABCDEFGHIJKLMNOPQRSTUVWZ', s[2:]):
          dims.append('dim{0}'.format(letter))
      if dims:
          return ' '.join(dims)
docs: Given a sorted list of subgroups, return a string appropriate to provide as
      the a composite track's `filterComposite` argument
  
      >>> import trackhub
      >>> trackhub.helpers.filter_composite_from_subgroups(['cell', 'ab', 'lab', 'knockdown'])
      'dimA dimB'
  
      Parameters
      ----------
      s : list
          A list representing the ordered subgroups, ideally the same list
          provided to `dimensions_from_subgroups`. The values are not actually
          used, just the number of items.