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group_purpose message event

A group purpose was updated

Compatibility: RTM

    "type": "message",
    "subtype": "group_purpose",
    "ts": "1358877455.000010",
    "user": "U2147483828",
    "purpose": "whatever",
    "text": "<@U2147483828|cal> set the group purpose: whatever"

A group_purpose message is sent when the purpose for a private group is changed using the group.setPurpose method.