    # 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