code: func NewCursor(ps []Prefix) *Cursor {
  	ps = newSortedPrefixes(ps, sortAscending, false)
  	if len(ps) == 0 {
  		return nil
  	}
  	c := &Cursor{ps: ps}
  	c.set(0, c.ps[0].IP.To16())
  	return c
  }
docs: // NewCursor returns a new cursor.