human
Solve the following python programming problem given the following code:
def arbitrary_phone_numbers_and_prefixes(country_code_and_prefixes) : 
    country_code_to_prefixes = { 
        
} 
    for (country_code, prefix) in country_code_and_prefixes : 
        if (country_code not in <MASK>) : 
            country_code_to_prefixes [country_code] = [] 
country_code_to_prefixes [country_code].append (prefix) 
for (country_code, prefix) in country_code_and_prefixes : 
        remainder_len = (10 - len (prefix)) 
        def _get_national_number() : 
            return (prefix + str (random.randint (((1 if prefix else 2) * (10 ** (remainder_len - 1))), ((10 ** remainder_len) - 1)))) 
while True : 
            national_number = _get_national_number () 
            if (not any (((national_number.startswith (another_prefix) and another_prefix.startswith (prefix)) for another_prefix in country_code_to_prefixes [country_code] if (another_prefix and (another_prefix != prefix))))) : 
                (yield ((country_code + national_number), prefix)) 
                break 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is Python program to fix + str (random.
country_code_to_prefixes