unsigned type is dangerous

google的tcmalloc代码里面有这样一段warning：

// NOTE: unsigned types are DANGEROUS in loops and other arithmetical

// places. Use the signed types unless your variable represents a bit
// pattern (eg a hash value) or you really need the extra bit. Do NOT
// use 'unsigned' to express "this value should always be positive";

// use assertions for this.

尽管一直知道这个，可就老是情不自禁的使用unsigned，太二了！

以后除非万不得已，肯定不用了。