def avoids(word, forbidden):
    """Returns True if word does not contain any letter in forbidden string

    >>> avoids('yummy', 'abcdefg')
    True
    >>> avoids('dictionary', 'abcdefg')
    False
    >>> avoids('crypt', 'aeiou')
    True
    >>> avoids('tangible', 'aeiou')
    False
    """
    for letter in word:
        if letter in forbidden:
            return False
    return True