看 atc15 论文的时候，算法描述看见一个 s.t. 符号，不明其意。后来在
stackexchange 上找到了答案:
Usually, the acronym s.t. means `such that'. In the context of optimization, it means
`subject to'. Also note that `such that' does *not* have the same meaning as `so that'.
- Such that, describes *how* something should be done.
- So that, describes *why* something should be done.
For clarity, it's usually best to avoid s.t. and simply write such that.