def append(a, vancestors):
    """
    Append ``a`` to the list of the virtual ancestors, unless it is already
    included.
    """
    add = True
    for j, va in enumerate(vancestors):
        if issubclass(va, a):
            add = False
            break
        if issubclass(a, va):
            vancestors[j] = a
            add = False
    if add:
        vancestors.append(a)
