Mermaid snippet:

    flowchart TB
    A --> C
    A --> D
    B --> C
    B --> D

More complex case:

    sequenceDiagram
    participant Alice
    participant Bob
    Alice->>John: Hello John, how are you?
    loop Healthcheck
    	John->>John: Fight against hypochondria
    end
    Note right of John: Rational thoughts
    prevail!
    John-->>Alice: Great!
    John->>Bob: How about you?
    Bob-->>John: Jolly good!