# A function to be tested.

def insert_after(L, n1, n2):
    """
    (list, int, int) -> NoneType
    
    After each occurrence of n1 in L, insert n2.
    
    >>> L = [5, 1, 2, 1, 6]
    >>> insert_after(L, 1, 99)
    >>> L
    [5, 1, 99, 2, 1, 99, 6]
    """
    
    i = 0
    while i < len(L):
        if L[i] == n1:
            L.insert(i+1, n2)
            i += 1
        i += 1

if __name__ == "__main__":
    import doctest
    doctest.testmod()