How to create all combinations from a nested list while preserving the structure using R?

Question

Given a nested list, how to create all possible lists from its elements, while preserving the structure of the nested list?

Nested list:

Answer 1

Combining Ben Nutzer's brilliant answer and Joris Chau's brilliant comment, the answer will become a one-liner:

apply(expand.grid(data.frame(l)), 1L, relist, skeleton = rapply(l, head, n = 1L, how = "list")) 

It creates a list of lists with as many elements as rows returned by expand.grid(). The result is better visualised by the output of str():

str(apply(expand.grid(data.frame(l)), 1L, relist, skeleton = rapply(l, head, n = 1L, how = "list")))

List of 16
 $ :List of 3
  ..$ a:List of 1
  .. ..$ b: num 1
  ..$ c:List of 1
  .. ..$ d:List of 2
  .. .. ..$ e: num 3
  .. .. ..$ f: num 5
  ..$ g: num 7
 $ :List of 3
  ..$ a:List of 1
  .. ..$ b: num 2
  ..$ c:List of 1
  .. ..$ d:List of 2
  .. .. ..$ e: num 3
  .. .. ..$ f: num 5
  ..$ g: num 7
...
...
...
 $ :List of 3
  ..$ a:List of 1
  .. ..$ b: num 2
  ..$ c:List of 1
  .. ..$ d:List of 2
  .. .. ..$ e: num 4
  .. .. ..$ f: num 6
  ..$ g: num 7