class Sass::Engine

def tree(arr, i = 0)
  return [], i if arr[i].nil?
  base = arr[i].tabs
  nodes = []
  while (line = arr[i]) && line.tabs >= base
    if line.tabs > base
      if line.tabs > base + 1
        raise SyntaxError.new("The line was indented #{line.tabs - base} levels deeper than the previous line.", line.index)
      end
      nodes.last.children, i = tree(arr, i)
    else
      nodes << line
      i += 1
    end
  end
  return nodes, i
end