Comment by PuercoPop

Does it? Asking for a value when encountering an unbound variable is a default restart

    $ sbcl
    This is SBCL 2.1.1.52.HEAD.321-f8a57bcca, an         
    implementation of ANSI Common Lisp.
    More information about SBCL is available at 
    <http://www.sbcl.org/>.

    SBCL is free software, provided as is, with absolutely no warranty.
    It is mostly in the public domain; some portions are provided under
    BSD-style licenses.  See the CREDITS and COPYING files in the
    distribution for more information.
    \* (\* x x)

    debugger invoked on a UNBOUND-VARIABLE in thread
    #<THREAD "main thread" RUNNING {1001860103}>:
      The variable X is unbound.

    Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

    restarts (invokable by number or by possibly-abbreviated name):
      0: [CONTINUE   ] Retry using X.
      1: [USE-VALUE  ] Use specified value.
      2: [STORE-VALUE] Set specified value and use it.
      3: [ABORT      ] Exit debugger, returning to top level.

    (SB-INT:SIMPLE-EVAL-IN-LEXENV X #<NULL-LEXENV>)

0] 2

    Enter a form to be evaluated: 3
    9
    \*