I didn't understand as much as I'd have liked of the linked article. Let me check something. In Mathematica, are the M expressions the internal 'FullForm' repre

I didn't understand as much as I'd have liked of the linked article. Let me check something. In Mathematica, are the M expressions the internal 'FullForm' representations of expressions? So, for example, I type: a = {1,2,3}; Which is represented internally as an 'M expression' of List[1,2,3] (obtainable through FullForm[a], or its head through Head[a])? Is that it? I didn't realise that was a named concept. It does make sense to have a completely unified internal representation underneath the syntactic sugar, since you'd need everything to be in that form for the term rewriting system to work. Thanks!