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;

theorem Th1:
  a is LTL-formula iff a in LTL_WFF
proof
  thus a is LTL-formula implies a in LTL_WFF
  proof
    assume a is LTL-formula;
    then a is Element of LTL_WFF by Def10;
    hence thesis;
  end;
  assume a in LTL_WFF;
  hence thesis by Def9,Def10;
end;
