1-Nitroso-1,7-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one; 1-nitroso-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one; 1-nitroso-1H-pyrazolo[3,4-d]pyrimidin-4(5H)-one