# File lib/active_support/multibyte/chars.rb, line 181
      def insert(offset, fragment)
        unpacked = self.class.u_unpack(@wrapped_string)
        unless offset > unpacked.length
          @wrapped_string.replace(
            self.class.u_unpack(@wrapped_string).insert(offset, *self.class.u_unpack(fragment)).pack('U*')
          )
        else
          raise IndexError, "index #{offset} out of string"
        end
        self
      end