It really helps if you post your code using code fences so the forum property formats it:
```
your code here
```
Two things with filter.
- I think you have to use square brackets, not parens
- Filter returns a List object containing all the Items that match the filter
So the line should be:
if(Sensor.members.filter[s|s.name == mm].size > 0) // there is at least one Item named mm