Sure, not everything is an expression; but an expressive language provides an expression that can contain other syntax classes and confine all their effects wit

Sure, not everything is an expression; but an expressive language provides an expression that can contain other syntax classes and confine all their effects within the bounds of the expression. E.g., SML's 'let' expression. In a language that doesn't allow that, we end up having to do some really weird things (https://github.com/yawaramin/lambdak https://github.com/yawaramin/lambdak).