reserve k,n,m for Nat,
  a,x,X,Y for set,
  D,D1,D2,S for non empty set,
  p,q for FinSequence of NAT;
reserve F,F1,G,G1,H,H1,H2 for LTL-formula;
reserve sq,sq9 for FinSequence;
reserve L,L9 for FinSequence;
reserve j for Nat;
reserve j1 for Element of NAT;

theorem
  F is_subformula_of H implies Subformulae F c= Subformulae H
proof
  assume
A1: F is_subformula_of H;
  let a be object;
  assume a in Subformulae F;
  then consider F1 such that
A2: F1 = a and
A3: F1 is_subformula_of F by Def24;
  F1 is_subformula_of H by A1,A3,Th35;
  hence thesis by A2,Def24;
end;
