This is a follow-up to this question.
I'd like to define
\fa to display
\forall when typed as usual but be
\fa is used as a subscript. For example, I want to write sentences like
Let $(\K_\fa, \leq_\fa)$ be the $\fa$-closure of ...
and have the first
\fa display in regular size but the second display using
Is there any easy way to do this?