theorem th14:
  {prop i} |- 'X' prop i & not {prop i} |-0 'X' prop i
proof
  prop i in {prop i} by TARSKI:def 1;then
  {prop i} |- prop i by LTLAXIO1:42;
  hence {prop i} |- 'X' prop i by LTLAXIO1:44;
  thus not {prop i} |-0 'X' prop i by th266,th262ac1;
end;
