def get_tag(expnum, key):
    """given a key, return the vospace tag value.

    @param expnum: Number of the CFHT exposure that a tag value is needed for
    @param key: The process tag (such as mkpsf_00) that is being looked up.
    @return: the value of the tag
    @rtype: str
    """

    uri = tag_uri(key)
    force = uri not in get_tags(expnum)
    value = get_tags(expnum, force=force).get(uri, None)
    return value