# File lib/mail/multibyte/chars.rb, line 122
        def insert(offset, fragment)
          unpacked = Unicode.u_unpack(@wrapped_string)
          unless offset > unpacked.length
            @wrapped_string.replace(
              Unicode.u_unpack(@wrapped_string).insert(offset, *Unicode.u_unpack(fragment)).pack('U*')
            )
          else
            raise IndexError, "index #{offset} out of string"
          end
          self
        end