@param s kstring to write into
@param n total number of items in @p a (<= 0 to encode BCF_BT_NULL)
@param a input data to encode
@param wsize vector length (<= 0 is equivalent to @p n)
@return 0 on success; < 0 on error
@note @p n should be an exact multiple of @p wsize
@param s kstring to write into @param n total number of items in @p a (<= 0 to encode BCF_BT_NULL) @param a input data to encode @param wsize vector length (<= 0 is equivalent to @p n) @return 0 on success; < 0 on error @note @p n should be an exact multiple of @p wsize