i =0 是这么执行的
$ ruby -e 'puts RubyVM::InstructionSequence.compile(%q~i=0~).disasm'
== disasm: RubyVM::InstructionSequence:compiled<@>==========
local table (size: 2, argc: 0 [opts: 0, rest: -1, post: 0, block: -1] s1)
[ 2] i
0000 trace 1 ( 1)
0002 putobject 0
0004 dup
0005 setlocal i
0007 leave