def swap(x, j, k):
    """ x is a list.
    swaps entries j and k in the list."""
    if j != k:
        x[j], x[k] = x[k], x[j]
def clear(x):
    x = []


x = [1,2,3,4]
print(x)
swap(x, 1,3)
print(x)
clear(x)
print(x)