def number_of_occurences(source_string, symbols=None):
    """
        function number_of_occurences()

        Count the number of time the symbol(s) occured in <source_string>.

        source_string   : str
        symbols         : list of strings
    """
    if source_string is None:
        return 0
    nbr = 0
    for char in source_string:
        if char in symbols:
            nbr += 1
    return nbr
