def formatter(data, headers):
    """Pretty-print a github notification."""
    # TODO: Write nice useful formatters. This is only an example.
    if headers["X-GitHub-Event"] == "push":
        pusher, ref, a, b, c = [
            data[k] for k in ["pusher", "ref", "after", "before", "compare"]
        ]
        pusher = f"[@{pusher['name']}](https://github.com/{pusher['name']})"
        data["body"] = f"{pusher} pushed on {ref}: [{b} → {a}]({c}):\n\n"
        for commit in data["commits"]:
            data["body"] += f"- [{commit['message']}]({commit['url']})\n"
    else:
        data["body"] = "notification from github"
    data["digest"] = headers["X-Hub-Signature-256"].replace("sha256=", "")
    return data