来自不同容器的元素的所有可能组合（每个容器一个元素）

我有一个列表，其中每个元素都是一组数字。所有集合的长度都不同：

I have a list, where each element is a set of numbers. Lengths of all sets are different:

 a <- list(1,c(2,3),c(4,5,6))
#> a
#[[1]]
#[1] 1

#[[2]]
#[1] 2 3

#[[3]]
#[1] 4 5 6 

I' d想从每个集合中获得一个元素的所有可能组合。在此示例中，应为：

I'd like to get all possible combinations of one element from each set. In this example it should be:

1 2 4，1 2 5，1 2 6，1 3 4，1 3 5，1 3 6

1 2 4, 1 2 5, 1 2 6, 1 3 4, 1 3 5, 1 3 6

我觉得这里* apply-functions的某些组合会很有用，但无法弄清楚该怎么做。

I feel that some combination of *apply-functions here would be useful, but can't figure out how to do that.