Speaking of speaking too soon, I may have said "user libraries can't get very far beyond [an fexpr language's core syntax]," but I want to add the disclaimer that there's no way I actually know that.
In fact, I was noticing that Penknife's parse/compile phase is a lot like fexpr evaluation. The operator's behavior is called with the form body, and that operator takes care of parsing the rest, just like an fexpr takes care of evaluating the rest. So I think a natural fexpr take on compiler techniques is just to eval code in an environment full of fexprs that calculate compiled expressions or static types. That approach sounds really familiar to me, so it probably isn't my idea. :-p
Sounds like transient symbols are essentially in a file-local namespace, which makes them lexically scoped (the lexical context being the file!), and that transient symbols are bound in the dynamic environment just like internal symbols are. So whenever lexical scope is needed, another file is used. Meanwhile, (====) can simulate a file break, making it a little less troublesome.
I think it works because while you're inside the let, you don't call anything that depends on a global function named x. :) That's in the FAQ too:
What happens when I locally bind a symbol which has a function definition?
That's not a good idea. The next time that function gets executed within the dynamic context the system may crash. Therefore we have a convention to use an upper case first letter for locally bound symbols:
(de findCar (Car List)
(when (member Car (cdr List))
(list Car (car List)) ) )