theorem Th119:
  for n st n > 0 holds (a followed_by b).n = b
proof
  let n;
A1: n in NAT by ORDINAL1:def 12;
  assume n > 0;
  hence (a followed_by b).n = (NAT --> b).n by Th31
    .= b by A1,FUNCOP_1:7;
end;
