reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem Th16:
  A c= 1 implies A = {} or A = 1
proof
  assume that
A1: A c= 1 and
A2: A <> {} and
A3: A <> 1;
  A c< 1 by A1,A3;
  hence contradiction by A2,Th14,ORDINAL1:11;
end;
