out-of-order: def foldLeft[B](z: B)(op: (A,B) => B): B

In https://docs.scala-lang.org/style/declarations.html
There is a out-of-order in MULTIPLE PARAMETER LISTS For type inference: 
```def foldLeft[B](z: B)(op: (A,B) => B): B``` should be changed to ```def foldLeft[B](z: B)(op: (B,A) => B): B```