class Fugit::Nat::SlotGroup

def determine_hms
  return [ [ [ '*' ], [ '*' ] ] ] if @hms.empty?
  hms = @hms.dup
    #
  while ig = (hms.count > 1 && hms.index { |hm| hm.graded? }) do
    sg = hms[ig]
    so = hms.delete_at(ig == 0 ? 1 : ig - 1)
    sg.append(so)
  end
  hms
    .collect(&:a)
    .inject({}) { |r, hm|
      hm[1].each { |m| (r[m] ||= []).concat(hm[0]) }
      r }
    .inject({}) { |r, (m, hs)|
      (r[hs.sort] ||= []) << m
      r }
    .to_a
end