relative_path_from

# File lib/pathname.rb, line 693
  def relative_path_from(base_directory)
    dest_directory = self.cleanpath.to_s
    base_directory = base_directory.cleanpath.to_s
    dest_prefix = dest_directory
    dest_names = []
    while r = chop_basename(dest_prefix)
      dest_prefix, basename = r
      dest_names.unshift basename if basename != '.'
    end
    base_prefix = base_directory
    base_names = []
    while r = chop_basename(base_prefix)
      base_prefix, basename = r
      base_names.unshift basename if basename != '.'
    end
    if dest_prefix != base_prefix
      raise ArgumentError, "different prefix: #{dest_prefix.inspect} and #{base_directory.inspect}"
    end
    while !dest_names.empty? &&
          !base_names.empty? &&
          dest_names.first == base_names.first
      dest_names.shift
      base_names.shift
    end
    if base_names.include? '..'
      raise ArgumentError, "base_directory has ..: #{base_directory.inspect}"
    end
    base_names.fill('..')
    relpath_names = base_names + dest_names
    if relpath_names.empty?
      Pathname.new('.')
    else
      Pathname.new(File.join(*relpath_names))
    end
  end