def dot_format(l):
    """
    Take a list l that is a factorial based number
    and returns it in dot format.
    
    i.e. [0, 2, 1] becomes 0.2.1
    """
    # empty list
    if len(l) < 1:
        return ""
    # start with just first element no dot
    dot_string = str(l[0])
    # add rest if any with dots
    for e in l[1:]:
        dot_string += "."+str(e)
        
    return dot_string