def is_no_cache_key_option(number):
    """Return ``True`` iff the option number identifies a NoCacheKey option.

    A :coapsect:`NoCacheKey option<5.4.2>` is one for which the value
    of the option does not contribute to the key that identifies a
    matching value in a cache.  This is encoded in bits 1 through 5 of
    the *number*.  Options that are :func:`unsafe<is_unsafe_option>`
    are always NoCacheKey options.
    """
    return (0x1c == (number & 0x1e))