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 Th5:
  H is next implies H = 'X' the_argument_of H
proof
  assume
A1: H is next;
  then not H is negative by Lm19;
  hence thesis by A1,Def18;
end;
