def after_prefix(string, prefix):
    """Return 'string' with 'prefix' removed.

    If 'string' does not start with 'prefix' then None is returned.

    Usage examples:

    >>> after_prefix('dog/cat/', 'dog/')
    'cat/'

    >>> after_prefix('dog/cat/', 'mouse/')

    :string: string to operate on
    :prefix: string prefix to remove
    :returns: string representing 'string' with 'prefix' removed or None

    """
    if string.startswith(prefix):
        return string[len(prefix):]
    return None