unsigned bit = mask&~(mask-1);
for(active_index=0; (bit>>active_index)>1; ++active_index) ;
acc.state.set((acc.state&~bit)|(acc.target&bit));
unsigned bit = mask&~(mask-1);
for(active_index=0; (bit>>active_index)>1; ++active_index) ;
acc.state.set((acc.state&~bit)|(acc.target&bit));