def generate_permutations(elements, n):
    """
    Heap's algorithm for generating all n! permutations in a list
    https://en.wikipedia.org/wiki/Heap%27s_algorithm

    """
    c = [0] * n
    yield elements
    i = 0
    while i < n:
        if c[i] < i:
            if i % 2 == 0:
                elements[0], elements[i] = elements[i], elements[0]
            else:
                elements[c[i]], elements[i] = elements[i], elements[c[i]]
            yield elements
            c[i] += 1
            i = 0
        else:
            c[i] = 0
            i += 1
