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;

theorem
  (G is negative or G is next) implies the_argument_of G is_subformula_of G
proof
  assume G is negative or G is next;
  then the_argument_of G is_immediate_constituent_of G by Th20,Th21;
  then the_argument_of G is_proper_subformula_of G by Th29;
  hence thesis;
end;
