从数据结构中间层中剔除值

Question

我有一个列表，其中每个项目是一个引用excel工作簿的列表。
在这些工作簿中，每个项目都是一个列表，指的是该工作簿中的工作表。
在这些工作表中，每个项目都是一个参考该工作表上的列名称的向量。
我只对每个向量中每个工作表的第一列的名称感兴趣

通过这些数字，我创建了一个新的结构，可以插入到数据框中

每行仍将引用工作表
点中的每个项目都是所有第一列名称的向量

由于要读取的数据类型的性质，粘贴样本数据有点困难，所以希望它从我的描述中足够清楚。

因此，如果l1代表上述列表，那么l2代表以下列表：

list(c((Worksheet1, Sheet1, A1), (Worksheet1, Sheet2, A1), (Worksheet1, Sheet3, A1)), c((Worksheet2, Sheet1, A1), (Worksheet2, Sheet2, A1), (Worksheet2, Sheet3, A1))，

从而返回左上角单元格的向量列表

我可以使用同心for循环来完成这个，但是我无法弄清楚在这里使用apply系列（或地图系列）的措辞。如何以这种方式跟踪列表？

我最好的尝试是利用我的知识（不只是吐出错误）

l2 <- lapply(l1, "[[", 1)

但是，它返回每个工作簿的第一张表中的所有列的向量，这不是我所追求的。

dput编辑：

structure(list(`Sheet 1` = structure(list(Sheet1 = c("Claim Status Report", 
"X__1", "Report A0817B"), Sheet2 = c("Standard Claims Lodgement-year Costs Breakdown Summary", 
"X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
"X__9", "X__10", "X__11", "X__12", "X__13", "X__14", "X__15", 
"X__16", "X__17", "X__18", "X__19"), Sheet3 = c("Standard Claims Lodgement-year Costs Breakdown by Claim", 
"X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
"X__9", "X__10", "X__11", "X__12", "X__13", "X__14", "X__15", 
"X__16", "X__17", "X__18", "X__19", "X__20", "X__21"), Sheet4 = "Claims with Statistical Case Estimate Variations over $1000", 
    Sheet5 = "Claims Received from  01/01/2018 to 31/01/2018", 
    Sheet6 = "Minor Claims received between 01/01/2018 and 31/01/2018", 
    Sheet7 = c("Medical Certificates Received between 01/01/2018 and 31/01/2018", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10", "X__11"), Sheet8 = c("All (net) Payments entered between 01/01/2018 and 31/01/2018", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10")), .Names = c("Sheet1", "Sheet2", "Sheet3", 
"Sheet4", "Sheet5", "Sheet6", "Sheet7", "Sheet8")), `Sheet 2` = structure(list(
    Sheet1 = c("Claim Status Report", "X__1", "Report A0817B"
    ), Sheet2 = c("Standard Claims Lodgement-year Costs Breakdown Summary", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10", "X__11", "X__12", "X__13", "X__14", "X__15", 
    "X__16", "X__17", "X__18", "X__19"), Sheet3 = c("Standard Claims Lodgement-year Costs Breakdown by Claim", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10", "X__11", "X__12", "X__13", "X__14", "X__15", 
    "X__16", "X__17", "X__18", "X__19", "X__20", "X__21"), Sheet4 = "Claims with Statistical Case Estimate Variations over $1000", 
    Sheet5 = c("Medical Certificates Received between 01/12/2017 and 31/12/2017", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10", "X__11"), Sheet6 = c("All (net) Payments entered between 01/12/2017 and 31/12/2017", 
    "X__1", "X__2", "X__3", "X__4", "X__5", "X__6", "X__7", "X__8", 
    "X__9", "X__10")), .Names = c("Sheet1", "Sheet2", "Sheet3", 
"Sheet4", "Sheet5", "Sheet6"))), .Names = c("Sheet 1", "Sheet 2"
))