theorem Th7:
  for x ex S st rng S = {x}
proof
  let x;
  consider f such that
A1: dom f = NAT and
A2: rng f = {x} by FUNCT_1:5;
  reconsider f as sequence of X by A1,A2,FUNCT_2:2;
  take f;
  thus thesis by A2;
end;
