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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def user_avatar_url(username, size=64, default="retro"):
 """Get the avatar URL of the provided Fedora username. The URL is returned from the Libravatar service. Args: username (str):
 The username to get the avatar of. size (int):
 Size of the avatar in pixels (it's a square). default (str):
 Default avatar to return if not found. Returns: str: The URL to the avatar image. """

    openid = "http://{}.id.fedoraproject.org/".format(username)
    return libravatar_url(openid=openid, size=size, default=default)