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1.7.1 Error Message for a Symbol Without a Function

When we evaluated fill-column to find its value as a variable, we did not place parentheses around the word. This is because we did not intend to use it as a function name.

If fill-column were the first or only element of a list, the Lisp interpreter would attempt to find the function definition attached to it. But fill-column has no function definition. Try evaluating this: 

(fill-column)

In GNU Emacs version 21, you will create a *Backtrace* buffer that says: 

---------- Buffer: *Backtrace* ----------
Debugger entered--Lisp error: (void-function fill-column)
  (fill-column)
  eval((fill-column))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------

(Remember, to quit the debugger and make the debugger window go away, type q in the *Backtrace* buffer.)

In GNU Emacs 20 and before, you will produce an error message that says: 

Symbol's function definition is void: fill-column

(The message will go away away as soon as you move the cursor or type another key.)