I am supposed to write a recursive function that takes a list of strings or a list of lists of strings and return the list without vowels, if found. Here is my attempt to solve it:

```
def noVow(seq):
    keys = ['a','i','e','o','u','u']
    if not seq or not isinstance(seq, list) :
        return 
    else:
        if seq[0]  in keys:
            del seq[0]
            return (noVow(seq[0:]))
        else:
            return (noVow(seq[1:]))

li = ["b", "c", "d","a"]
print (noVow(li))

```

I am aware that the bug lies in my base case however I can't come up with the right base case. 

Note that the recursive function has to be written in pure functional programming i.e. side effects are not allowed.