The @on decorator is used to define the event that will trigger the automation. 
@on("github.pull_request")
The @on("event") decorator will correspond to the event payload of your function. See the events documentation for more information.

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Arguments

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event
string
required
The event that will trigger the automation. Supported values are github.issue_comment, github.issues, github.pull_request, github.pull_request_review, github.pull_request_review_comment, github.pull_request_comment, schedule.daily, schedule.weekly, webhook.See the events documentation for more information on the payloads for each event.