theorem Th24: p => p is ctaut
  proof
    let g;
    set v = VAL g;
A1: v.p = 1 or v.p = 0 by XBOOLEAN:def 3;
    thus v.(p => p) = (v.p => v.p) by LTLAXIO1:def 15
    .= 1 by A1;
  end;
