分支：
run keyword if | condition | do something
... | else if | condition | do something
... | else | condition | do something
循环1：
:FOR | ${item} | IN | ${item1} | ${item2} | ...
| do something
循环2：
:FOR | ${item} | IN RANGE | ${range1} | [ ${range2}]
| do something
注意：${range2}是可选的，如果没有${range2}，则范围设为 0~${range1}
循环3：
:FOR | ${item} | IN | @{arglist}
| do something
从变量列表中获取每个变量
