# File lib/haml/util.rb, line 219
    def lcs(x, y, &block)
      x = [nil, *x]
      y = [nil, *y]
      block ||= proc {|a, b| a == b && a}
      lcs_backtrace(lcs_table(x, y, &block), x, y, x.size-1, y.size-1, &block)
    end