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;

theorem Th20:
  F is negative implies (H is_immediate_constituent_of F iff H =
  the_argument_of F)
proof
  assume F is negative;
  then F = 'not' the_argument_of F by Def18;
  hence thesis by Th13;
end;
