RF_D
RF_AD = (RF_A | RF_D),
RF_BD = (RF_B | RF_D),
RF_CD = (RF_C | RF_D),
RF_ABD = (RF_A | RF_B | RF_D),
RF_ACD = (RF_A | RF_C | RF_D),
RF_BCD = (RF_B | RF_C | RF_D),
RF_ABCD = (RF_A | RF_B | RF_C | RF_D),
return RF_D;