> Your proposal, on the other hand, is as far as I can tell quite a bit more wide-reaching. Please correct me if I've misunderstood, but aren't you proposing that all property methods accept region indices rather than cells?

that's correct: The current interface is not cell specific, and I _strongly_ propose propose to keep it that way. (the fact that the arrays which are currently passed to the computation methods are usually for cells is just a coincident because the opm-* modules do not currently support any discretization other than cell-centered finite volumes.)

> ```c++
void viscosity(int n, const double* p, const double* z, const int* cells,
               double* mu, double* dmudp);
```

there currently is no 'cells' argument on this level. I propose to add an array for the pvt quantities which transforms the signature from
```c++
void viscosity(const int n, const double* p, const double* z,
               double* mu, double* dmudp)
```
 to

```c++
void viscosity(const int n, const int* pvtRegionIdx, const double* p, const double* z,
               double* mu, double* dmudp)
```

(note that the current interface nowhere requires that the 'n' quantities to be calculated correspond to cells.)