code: function find(b, ptr, target, level, compare, gt) {
    level = level || r_levels(b, ptr)
    compare = compare || _compare
    gt = gt !== false
    var prev_ptr = 0
    while(true) {
      if(level < 0) break;
      //if this level is last item
      var next_ptr = r_level(b, ptr, level)
      if(next_ptr === 0)
        level --
      else {
        var cmp = compare(r_value(b, next_ptr), target, b)
        if(gt ? cmp > 0 : cmp >= 0)
          level --
        else {
          prev_ptr = ptr
          ptr = next_ptr
        }
      }
    }
  
    return ptr //gt ? ptr : prev_ptr
  }
docs: /*
  ranges
  
  cmp > 0 // get last equal element
  cmp >= 0 // get last smaller element
  to get first gt element, use find(>) + 1?