reserve k,n for Element of NAT,
  a,Y for set,
  D,D1,D2 for non empty set,
  p,q for FinSequence of NAT;

theorem Th1:
  a is CTL-formula iff a in CTL_WFF
proof
  thus a is CTL-formula implies a in CTL_WFF
  proof
    assume a is CTL-formula;
    then a is Element of CTL_WFF by Def13;
    hence thesis;
  end;
  assume a in CTL_WFF;
  hence thesis by Def12,Def13;
end;
