class Sass::Selector::Sequence

def must_unify?(seq1, seq2)
  unique_selectors = seq1.map do |sseq|
    next [] if sseq.is_a?(String)
    sseq.members.select {|sel| sel.unique?}
  end.flatten.to_set
  return false if unique_selectors.empty?
  seq2.any? do |sseq|
    next false if sseq.is_a?(String)
    sseq.members.any? do |sel|
      next unless sel.unique?
      unique_selectors.include?(sel)
    end
  end
end