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 Th8:
  H is Until implies H = (the_left_argument_of H) 'U' (
  the_right_argument_of H)
proof
  assume
A1: H is Until;
  then ex H1 st H = H1 'U' the_right_argument_of H by Def20;
  hence thesis by A1,Def19;
end;
