theorem th18:
  F |- ('G' A) => A
  proof
A1: {A} c= F \/ {A} by XBOOLE_1:7;
    A in {A} by TARSKI:def 1;then
    F \/ {A} |- A by A1,LTLAXIO1:42;
    hence F |- ('G' A) => A by LTLAXIO1:57;
  end;
