局部迭代会重复调用用户定义的BOOST_PP_LOCAL_MACRO宏，它的参数是 macro boost local迭代指标（iteration index）。因为我们已经定义了TINY_size宏，所以只 index 已经我们需让BOOST_PP_MACRO调用它就行了。
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An iteration index is also maintained when iterating through the elements of a collection.
进行数字迭代的情形一样，在迭代集合中的元素时同样要维护一个迭代索引。
They specify the initial value of the index for the iteration and the index value at which iteration should cease, respectively.
它们分别指定迭代索引的初始值以及迭代索引的终止值。
If the var attribute is specified, then a scoped variable with the indicated name will be created and assigned the current value of the index for each pass through the iteration.
如果指定了var属性，那么将会创建一个带有指定名称的并限定了作用域的变量，并将每次迭代的当前索引值赋给该变量。
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