theorem Th42:
  for x being object holds (p^<*x*>).(len p + 1)=x
proof let x be object;
  dom <*x*> = Seg 1 by Def8;
  then 1 in dom <*x*>;
  hence (p^<*x*>).(len p + 1) = <*x*>.1 by Def7
    .=x;
end;
