```
import scala.collection.mutable.Map

var overlapByY: Map[Int, Int] = (0 to records.length).map(a => (a, 0)).toMap

```

Gives the error

```
polymorphic expression cannot be instantiated to expected type;
[error]  found   : Map          (in scala.collection.immutable)
[error]  required: Map[Int,Int] (in scala.collection.mutable)
[error]             var overlapByY: Map[Int, Int] = (0 to 8).map(a => (a, 0)).toMap
[error]                                                                       ^

```