uint prev(uint start); // gives the largest index i<=start such that IsBitSet(i)=true
uint prev2(uint start); // gives the largest index i<=start such that IsBitSet(i)=true
uint next(uint start); // gives the smallest index i>=start such that IsBitSet(i)=true
uint prev(uint start); // gives the largest index i<=start such that IsBitSet(i)=true
uint prev2(uint start); // gives the largest index i<=start such that IsBitSet(i)=true
uint next(uint start); // gives the smallest index i>=start such that IsBitSet(i)=true