Tucked right behind my passport, I carry a page which is torn out of *A brochure. Very explicitly, it states that on your day of travel, if you are in F/C, you can use any *A F/C lounge of any *A airline. I show that to the matrons and .... sometimes with a shrug, sometimes with a phone call to check with a super ... I always get in!

Yet, I know the LH F/C Terminals in FRA and MUC are off-limits unless on LH, but *A passengers can still use the F/C lounges in the main terminals.