Get generators working in an src block?

[George Mauer <gmauer@gmail.com>]

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I prefer to use source blocks for pretty much all my language learning.

I am trying to figure out generators in emacs-lisp but can't seem to run it
in a block.

I have

```
    #+begin_src emacs-lisp :results output
      (setq lexical-binding t)
      (require 'generator)

      (iter-defun my-iter (x)
        (let (val (iter-yield (1+ x)))
          (iter-yield (1+ val)))
        -1)

      (let* ((it1 (my-iter 5))
             (it2 (my-itr 0)))
        (print (iter-next it1))
        (print (iter-next it1 8))
        (print (iter-next it2 nil))
        (condition-case x
            (iter-next it1)
          (iter-end-of-sequence
           (print (cdr x)))))
    #+end_src
```

But running that gives me

> executing Emacs-Lisp code block...
> cl--assertion-failed: Assertion failed: lexical-binding

I thought that setting lexical-binding to t was enough, but apparently
something else is needed?

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