create a list of list
l <- list(a=list(1,2),b=list(3,4))
str(l)
List of 2
 $ a:List of 2
  ..$ : num 1
  ..$ : num 2
 $ b:List of 2
  ..$ : num 3
  ..$ : num 4


str(l[1]) and str(l$a) should have same structure BUT

str(l[1])
List of 1
 $ a:List of 2
  ..$ : num 1
  ..$ : num 2

str(l$a)
List of 2
 $ : num 1
 $ : num 2