
theorem Th17a:
  for p,q be boolean object holds
    ('not' q => 'not' p) => (('not' q => p) => q) = TRUE
  proof
    let p,q be boolean object;
A1: p = FALSE or p = TRUE by XBOOLEAN:def 3;
    q = FALSE or q = TRUE by XBOOLEAN:def 3;
    hence thesis by A1;
  end;
