如何在数据表中“取消列出”列

在我的表格中，某些单元格是向量而不是单个值，即列是列表而不是向量：

in my table, some cells are vectors instead of single value, i.e. the column is a list instead of vector:

dt1 <- data.table(
  colA=   c('A1','A2','A3'), 
  colB=list('B1',c('B2a','B2b'),'B3'),
  colC=   c('C1','C2','C3'), 
  colD=   c('D1','D2','D3')
)

dt1
#   colA    colB colC colD
#1:   A1      B1   C1   D1
#2:   A2 B2a,B2b   C2   D2
#3:   A3      B3   C3   D3 

我需要将其重塑为长格式，以取消列出该列 colB 。到目前为止，我是这样的：

I need to reshape it to a long format unlisting that column colB. So far I do it like this:

dt1[,.(colB=unlist(colB)),by=.(colA,colC,colD)]
#   colA colC colD colB
#1:   A1   C1   D1   B1
#2:   A2   C2   D2  B2a
#3:   A2   C2   D2  B2b
#4:   A3   C3   D3   B3

它确实可以胜任，但我不喜欢必须在 by = 中明确指出所有其他列名称。有更好的方法吗？

（我确信它已经在其他地方得到解答了，但到目前为止我找不到）

it does the job but I don't like that I have to indicate all other column names explicitly in by=. Is there better way to do this?
(I'm sure it's already answered elsewhere but I couldn't find it so far)

P.S。理想情况下，我想在没有任何外部软件包的情况下进行管理

P.S. ideally I would like to manage without any external packages