Take a nested list and return a single flattened list with all values except nil/null.
The challenge is to write a function that accepts an arbitrarily-deep nested list-like structure and returns a flattened structure without any nil/null values.
You can think of this data structure as a
Rose Tree. You are given a data type
'a tree that represents this data structure.
List [Elem 1, List [Elem 2, Elem 3, Empty, Elem 4], List [Empty], Elem 5]