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