List must only consists of specific value

[Ricky_Chua]

I want to make an expression that gives me the following

{"a", "a", "a"} = TRUE (ie: every element in the list must be "a")

{"a", "a", "b"} = FALSE (contains something other than "a")

{"a", "b", "c"} = FALSE (contains something other than "a")

In other words, I want an expression that checks the list contains anything other than "a", then apply NOT() to the expression. I thought of NOT(CONTAINS(...)), but that is not correct.