docs: 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