def METHOD_NAME(self):
    """
    `border` property getter. Return the common value of all side
    borders if they are identical. Otherwise return None.
    """
    found = None
    for side in ['top', 'right', 'bottom', 'left']:
        if not hasattr(self, "border_" + side):
            return
        old, found = found, getattr(self, "border_" + side)
        if found is None or (old is not None and found != old):
            return
    return found