┌─────┐                 ┌───┐
         │Alice│                 │Bob│
         └──┬──┘                 └─┬─┘
from start to actor                │
       ────>│                      │
            │                      │
            from start to actor    │
       ───────────────────────────>│
            │                      │
            │             to actor │
            │            ─────────>│
            │                      │
            │    from actor to end │
            │──────────────────────────>
            │                      │
            │from actor            │
            │───────────>          │
            │                      │
            │ from actor to actor  │
            │─────────────────────>│
            │                      │
         ┌──┴──┐                 ┌─┴─┐
         │Alice│                 │Bob│
         └─────┘                 └───┘