> I didn't see if the language supports a fluid let (any variable name is looked up at run time when the reference is encountered thru the stack levels until the name is found. [...]

These are called dynamic variables in Lisp. They're only really useful as a way of doing things like I/O redirection, and you can just shallow-bind them to make them fast (but then they need to be per-thread as well, and pushing a binding requires unwind-protect to pop on unwind).

> The other choice would be that the variables are lexically scoped. [...]

That's what the README says Fe does.