什么是时序图？

Sequence Diagrams show elements as they interact over time and they are organized according to object (horizontally) and time (vertically)

水平方向是角色，竖直方向是时间，这些元素交互是基于时间的

参考：https://www.visual-paradigm.com/guide/uml-unified-modeling-language/what-is-sequence-diagram/

角色有哪些？

Actor （参与者）

Lifeline（生命线）

Activation（活动）

Call Message （请求消息）

Return Message（响应消息）

Self Message（内部消息）

Operator	Fragment Type
alt	Alternative multiple fragments: only the one whose condition is true will execute.
opt	Optional: the fragment executes only if the supplied condition is true. Equivalent to an alt only with one trace.
par	Parallel: each fragment is run in parallel.
loop	Loop: the fragment may execute multiple times, and the guard indicates the basis of iteration.
region	Critical region: the fragment can have only one thread executing it at once.
neg	Negative: the fragment shows an invalid interaction.
ref	Reference: refers to an interaction defined on another diagram. The frame is drawn to cover the lifelines involved in the interaction. You can define parameters and a return value.
sd	Sequence diagram: used to surround an entire sequence diagram.