bug#748: Elisp: lexical-let and cyclic structures

[Michael Heerdegen]

When using the reader constructs `#N=' and `#N#' for cyclic
structures, lexical-let sometimes produces errors which don't occur
with let.

Example: Eval the following:

(defun f (start)
  (lexical-let (start start)         ;; return a closure
    #1=(lambda (x) (if (= x start) x
                     (+ x (#1# (1- x)))))))

Then evaluating f ends with an error:

   (f 3)

    "Variable binding depth exceeds max-specpdl-size"


The problem also occurs without lambdas. Examples:

   (let         () '#1=(#1#))               ;; ==> (#0)
   (lexical-let () '#1=(#1#))               ;;  Error
   (let ((x '#1=(#1#))) (lexical-let () x)) ;; ==> (#0)
   `#1=(#1#)                                ;;  Error