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;
reserve j,j1 for Element of NAT;

theorem Th78:
  G in Subformulae H implies G is_subformula_of H
proof
  assume G in Subformulae H;
  then ex F st F = G & F is_subformula_of H by Def42;
  hence thesis;
end;
