a-foundation/src/main/java/com/ajjpj/afoundation/collection/ACollectionHelper.java
public static <T> List<T> flattenList(Iterable<? extends Iterable<T>> coll) {
        final List<T> result = new ArrayList<>();
        for(Iterable<T> o: coll) {
            for(T el: o) {
                result.add(el);
            }
        }
        return result;
    }
public static <T> List<T> flattenList(Iterable<? extends Iterable<T>> coll) {
        final List<T> result = new ArrayList<>();
        for(Iterable<T> o: coll) {
            for(T el: o) {
                result.add(el);
            }
        }
        return result;
    }
Takes a collection of collections and creates a new collection from the elements, leaving out the innermost
level of collection.