### Checklist
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* [ ] **Visual demo** attached
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Closes #141
Proposed Changes
Boundary events now requires label
### Checklist To ensure you provided everything we need to look at your PR: * [ ] **Brief textual description** of the changes present * [ ] **Visual demo** attached * [ ] **Steps to try out** present, i.e. [using the `@bpmn-io/sr` tool](https://github.com/bpmn-io/sr) * [ ] Related issue linked via `Closes {LINK_TO_ISSUE}` or `Related to {LINK_TO_ISSUE}`