I happeneed to see the javadoc of flatmap function and come across "evidence$4" - what is the purpose of it.

public <U> RDD<U> flatMap(scala.Function1<T,scala.collection.TraversableOnce<U>> f,
                          scala.reflect.ClassTag<U> evidence$4)

I had used this flatmap function by passing a function but didn't use "evidence$4" anytime.