Font dependency specification

[marta zhango]

I want to describe that font attributes are dependent upon three things -

 fontset, faces, and overlays.  How can I write this in TLA?

Perhaps like this

FontAttris(point) \def

\E face \in ActiveFaces(point),

\E overlay \in OverlaysAt(point),

\E charset \in Fontset(point)

or

FontFaceAttributes(font-at(point)) = faces(pt) ^ Overlays(pt) ^ Fontset(pt) 

or 

 □ (FontAttrs(pt) ∈ Faces(pt) × Overlays(pt) × Fontset(pt))

How can I approach this?