The invention relates to compound of the formula I or a salt thereof, wherein the substituents are as defined in the specification; to its preparation, to its use as medicament and to medicaments comprising it.本發明係關於一種式I化合物或其鹽,其中取代基係如說明書中所定義;其製備,其作為藥品之用途及包含該化合物或其鹽之藥品。