reserve A,B,C for Ordinal;
reserve a,b,c,d for natural Ordinal;
reserve l,m,n for natural Ordinal;
reserve i,j,k for Element of omega;
reserve x,y,z for Element of RAT+;

theorem Th30:
  not ex i,j being set st [i,j] is Ordinal
proof
  given i,j being set such that
A1: [i,j] is Ordinal;
  {} in {{i},{i,j}} by A1,ORDINAL3:8;
  hence thesis by TARSKI:def 2;
end;
