func (b *block) insert(i uint) bool {
	w, mask := wordMask(i)
	if b.bits[w]&mask == 0 {
		b.bits[w] |= mask
		return true
	}
	return false
}
insert sets the block b's ith bit and
returns true if it was not already set.
package intsets  
import (
	"bytes"
	"fmt"
)
type Sparse struct {
	root block
}
type word uintptr
const (
	_m            = ^word(0)
	bitsPerWord   = 8 << (_m>>8&1 + _m>>16&1 + _m>>32&1)
	bitsPerBlock  = 256  
	wordsPerBlock = bitsPerBlock / bitsPerWord
)
const (
	MaxInt = int(^uint(0) >> 1)
	MinInt = -MaxInt - 1
)
type block struct {
	offset     int                  
	bits       [wordsPerBlock]word  
	next, prev *block               
}
func wordMask(i uint) (w uint, mask word) {
	w = i / bitsPerWord
	mask = 1 << (i % bitsPerWord)
	return
}