Hi!

I have some Rcpp code to fill entries in a matrix, here's a MWE of what it does:

```
#include <Rcpp.h>
using namespace Rcpp;
void invert_tri(NumericMatrix &M, int K) {
for(int i=0 ; i<K ; ++i) {
for(int j=i+1 ; j<K ; ++j) {
M(j, i) = M(i, j);
}
}
}
// [[Rcpp::export]]
NumericMatrix multiply_entries(NumericMatrix M, int factor) {
int K = M.ncol();
for(int i=0 ; i<K ; ++i) {
for(int j=0 ; j<K ; ++j) {
M(i, j) = M(i, j) * factor;
}
}
invert_tri(M, K);
return M;
}
```

Then in R I would use it like this:

```
M = matrix(c(1,NA,2,4), nrow = 2, ncol = 2)
multiply_entries(M, 2L)
```

This changes

```
[,1] [,2]
[1,] 1 2
[2,] NA 4
```

into

```
[,1] [,2]
[1,] 2 4
[2,] 4 8
```

Then, with cpp11 I would sketch the following code

```
#include <cpp11.hpp>
#include <cpp11/doubles.hpp>
using namespace cpp11;
void invert_tri(writable::doubles_matrix<> &M, int K) {
for(int i=0 ; i<K ; ++i) {
for(int j=i+1 ; j<K ; ++j) {
M(j, i) = M(i, j);
}
}
}
[[cpp11::register]]
doubles_matrix<> multiply_entries(writable::doubles_matrix<> M, int factor) {
int K = M.ncol();
for(int i=0 ; i<K ; ++i) {
for(int j=0 ; j<K ; ++j) {
M(i, j) = M(i, j) * factor;
}
}
invert_tri(M, K);
return M;
}
```

But here doing `M(j, i) += M(i, j)`

would fail and with `=`

fails because of implicit deletion. Do you have any ideas for an alternative approach to translate the working code.