我写的命令：

egen 年份 =  group (year)

sem (a -> a1, ) (a -> a2, ) (a -> a3, ) (a -> a4, ) (a -> b, ) (a -> c, ) (b -> b1, ) (b ->b2, ) (b -> b3, ) (b -> b4, ) (b -> c, ) (c -> c1, ) (c -> c2, ) (c ->c3, ), method(mlmv) latent(a b c ), group (年份) ginvariant (none)

结果显示：invalid ‘group’

请问是我的命令有错误么？为什么显示 invalid ‘group’ 呢？