reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;
reserve sq,sq9 for FinSequence;
reserve L,L9 for FinSequence;

theorem Th59:
  H is_subformula_of H
proof
  take 1 , <*H*>;
  thus 1 <= 1;
  thus len <*H*> = 1 by FINSEQ_1:40;
  thus <*H*>.1 = H & <*H*>.1 = H;
  thus thesis;
end;
