Bidirectional Arrows

Arrows in both directions: `<-->`, `<-.->`, `<==>`.

bidirectional-arrows.mmd
graph LR
  A[Client] <-->|sync| B[Server]
  B <-.->|heartbeat| C[Monitor]
  C <==>|data| D[Storage]
Output
Flowchart, Zombie Mermaid sync heartbeat data Client Server Monitor Storage

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