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我是R的新手.我t1在R中有一个列表 ,看起来像

[[1]]
[[1]][[1]]
[1] "a"       "control"


[[2]]
[[2]][[1]]
[1] "a"        "disease1"


[[3]]
[[3]][[1]]
[1] "a"        "disease2"


[[4]]
[[4]][[1]]
[1] "b"       "control"


[[5]]
[[5]][[1]]
[1] "b"        "disease1"


[[6]]
[[6]][[1]]
[1] "b"        "disease2"

我需要从矢量中获得一个唯一的第一个元素列表,即["a","b"] t1.我怎样才能做到这一点？