class Sass::Selector::Sequence

def subweave(seq1, seq2)
  return [seq2] if seq1.empty?
  return [seq1] if seq2.empty?
  seq1, seq2 = seq1.dup, seq2.dup
  return unless (init = merge_initial_ops(seq1, seq2))
  return unless (fin = merge_final_ops(seq1, seq2))
  # Make sure there's only one root selector in the output.
  root1 = has_root?(seq1.first) && seq1.shift
  root2 = has_root?(seq2.first) && seq2.shift
  if root1 && root2
    return unless (root = root1.unify(root2))
    seq1.unshift root
    seq2.unshift root
  elsif root1
    seq2.unshift root1
  elsif root2
    seq1.unshift root2
  end
  seq1 = group_selectors(seq1)
  seq2 = group_selectors(seq2)
  lcs = Sass::Util.lcs(seq2, seq1) do |s1, s2|
    next s1 if s1 == s2
    next unless s1.first.is_a?(SimpleSequence) && s2.first.is_a?(SimpleSequence)
    next s2 if parent_superselector?(s1, s2)
    next s1 if parent_superselector?(s2, s1)
    next unless must_unify?(s1, s2)
    next unless (unified = Sequence.new(s1).unify(Sequence.new(s2)))
    unified.members.first.members if unified.members.length == 1
  end
  diff = [[init]]
  until lcs.empty?
    diff << chunks(seq1, seq2) {|s| parent_superselector?(s.first, lcs.first)} << [lcs.shift]
    seq1.shift
    seq2.shift
  end
  diff << chunks(seq1, seq2) {|s| s.empty?}
  diff += fin.map {|sel| sel.is_a?(Array) ? sel : [sel]}
  diff.reject! {|c| c.empty?}
  Sass::Util.paths(diff).map {|p| p.flatten}.reject {|p| path_has_two_subjects?(p)}
end