What do you say when someone says semper fi?

semper fidelis (shortened semperfi) is the motto of the United States Marine Corps. Translated it means always faithful and is used more or less as a greeting or term of endearment between marines. One marine sees another it is acceptable to say many things, one of which could be semper fi. For family members and retirees the phrase is a great way of distinguishign your self as someone who is still connected to the corps. For example the mother of marine approaches a retired vet and says my son is serving in the corps in which the vet could respond semper fi ma'am. An acceptable response would simply be semper fi, or semper (always faithful, or always) or any other term or greeting.