InterfaceDefList : InterfaceDef
                     | InterfaceDefList InterfaceDef

def p_InterfaceDefList(p):
    '''
    InterfaceDefList : InterfaceDef
                     | InterfaceDefList InterfaceDef
    '''
    if len(p) < 3:
        p[0] = InterfaceDefList(None, p[1])
    else:
        p[0] = InterfaceDefList(p[1], p[2])