Tuple selector sections

In Silver, one can write
```
map((.pp), l)
```
to get a list containing the value of the `pp` attribute for each tree in the list `l`.

One should analogously be able to write
```
map((.3), l)
```
for a list of tuples `l` to get a list of the third elements from all the tuples, but tuple selector sections like this are not currently implemented.