Python Sum of nested list with str items

Given: [[[[[[[[['A']]]]],[[[[['B']]]]],['C'],[[[[['D']]]]]]

Or something like that. A complicated nested list.

How can I return either: 'ABCD' or ['ABCD']

Recursive function with int is simple enough due to the sum helper function, but I can't imagine how to do this with str. Thanks for your help!