I have a list with two elements, where I have a variable of interest `value` for each combination of variables `country` and `gender`.

```
# Toy example
A <- list()

A[[1]] <- data.frame(country=c("Spain", "Spain", "France", "France"),
                       gender= c("M", "F", "M", "F"),
                       value = c(100,125,10,200))

A[[2]] <- data.frame(country=c("Spain", "Spain", "France"),
                       gender=c("M", "F", "F"),
                       value = c(150,75,100)) 

```

Data looks like this:

```
[[1]]
  country gender value
1   Spain      M   100
2   Spain      F   125
3  France      M    10
4  France      F   200

[[2]]
  country gender value
1   Spain      M   150
2   Spain      F    75
3  France      F   100

```

I would like to compute the mean of `value` (across the elements of the list) for each possible combination of `gender` and `country` taking into account that no all the combinations are in all the elements of the list (in that example, the second element has not a value for Males in France.

What I expect is something like this:

```
  country gender value
1   Spain      M   125
2   Spain      F   100
3  France      M     5
4  France      F   150 

```

Any idea how to deal with that?