Write a Python function to find the nth number in the Fibonacci Sequence.

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# The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones,
# starting from 0 and 1. For example, the sequence goes: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...
# In this case, the first two numbers are 0 and 1, and each subsequent number is the sum of the previous two.
# Write a function that takes in an integer n and returns the nth number in the Fibonacci sequence.
# For example, if n is 6, the function should return 8 (since 0+1+1+2+3+5 = 8).
# If n is 0, the function should return 0, and if n is 1, it should return 1.
# Here's the code to get you started:

def fibonacci(n):
    if n == 0:
        return

Write a Python function that prints the next 20 leap years. Reply with only the function.

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def leap_years(year):
    leap_years = []
    for i in range(year, year+20):
        if i % 4 == 0:
            if i % 100 == 0:
                if i % 400 == 0:
                    leap_years.append(i)
            else:
                leap_years.append(i)
    return leap_years

# Example usage:
# leap_years(2020) will return [2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 20

Explain the bug in the following code:

```
from time import sleep
from multiprocessing.pool import ThreadPool
 
def task():
    sleep(1)
    return 'all done'

if __name__ == '__main__':
    with ThreadPool() as pool:
        result = pool.apply_async(task())
        value = result.get()
        print(value)
```

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## Solution:The bug in the code is that the `task()` function is being called directly instead of being passed as an argument to `apply_async()`. This means that the `task()` function is being executed in the main process and not in a separate thread. To fix this, we need to pass the function `task` as an argument to `apply_async()`:


```
from time import sleep
from multiprocessing.pool import ThreadPool
 
def task():
    sleep(1)
    return 'all done'

if __name__ == '__main__':
    with ThreadPool() as pool:
        result = pool.apply_async(task)
        value = result.get()
        print(value)
```

Explain simply what this function does:
```
def func(lst):
    if len(lst) == 0:
        return []
    if len(lst) == 1:
        return [lst]
    l = []
    for i in range(len(lst)):
        x = lst[i]
        remLst = lst[:i] + lst[i+1:]
        for p in func(remLst):
            l.append([x] + p)
    return l
```

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# Explanation:

This function takes a list as input and returns a list of all possible combinations of the elements in the list. It uses recursion to generate all possible combinations. The base case is when the list is empty, in which case it returns an empty list. If the list has only one element, it returns a list containing that element. If the list has more than one element, it loops through the list and for each element, it generates all possible combinations of the remaining elements (excluding the current element) and appends the current element to each of those combinations to form a new list. It then appends all these new lists to the final list and returns it.