_layout_for_option

# File actionview/lib/action_view/layouts.rb, line 381
    def _layout_for_option(name)
      case name
      when String     then _normalize_layout(name)
      when Proc       then name
      when true       then Proc.new { _default_layout(true)  }
      when :default   then Proc.new { _default_layout(false) }
      when false, nil then nil
      else
        raise ArgumentError,
          "String, Proc, :default, true, or false, expected for `layout'; you passed #{name.inspect}"
      end
    end