Skeleton function: (LAMBDA-CLOSURE () () () (&OPTIONAL &REST REST)
                      (LET* ()
                        (LABELS ()
                          (BLOCK SETUPSTATEMENTS NIL)
                          (COND (T NIL)))))

Name of function is CLOSURE
   paramList of function is (&OPTIONAL &REST REST)
   env of function is NIL
   body of function is ((LET* ()
                          (LABELS ()
                            (BLOCK SETUPSTATEMENTS NIL)
                            (COND (T NIL)))))

Analyzing closure of form fGenFunc
   env vars of function are NIL
   params of function are (&OPTIONAL &REST REST)
   local vars of function are NIL
   methods of function are NIL
   setup block of function is (BLOCK SETUPSTATEMENTS NIL)
   cond of function is (COND (T NIL))

Now, adding local variable (x 2), using append

   an optional parameter (L nil), using insertNth

   a print statement in the block instead of the nil, using replaceNth

   a cond case to check if L is a list, using insertNth

and a method to compute x^2, using insertFromEnd

The resulting function is:
(LAMBDA-CLOSURE () () () (&OPTIONAL (L NIL) &REST REST)
  (LET* ((X 2))
    (LABELS ((SQUARE NIL (* X X)))
      (BLOCK SETUPSTATEMENTS
        (FORMAT T x is ~A, x^2 is ~A, L is ~A~% X (SQUARE) L))
      (COND ((LISTP L) (FORMAT T ~A is a list~% L)) (T NIL)))))

Analyzing f gives:

Name of function is CLOSURE
   paramList of function is (&OPTIONAL (L NIL) &REST REST)
   env of function is NIL
   body of function is ((LET* ((X 2))
                          (LABELS ((SQUARE NIL (* X X)))
                            (BLOCK SETUPSTATEMENTS
                              (FORMAT T x is ~A, x^2 is ~A, L is ~A~% X
                                      (SQUARE) L))
                            (COND
                              ((LISTP L) (FORMAT T ~A is a list~% L))
                              (T NIL)))))

Analyzing closure of form fGenFunc
   env vars of function are NIL
   params of function are (&OPTIONAL (L NIL) &REST REST)
   local vars of function are ((X 2))
   methods of function are ((SQUARE NIL (* X X)))
   setup block of function is (BLOCK SETUPSTATEMENTS
                                (FORMAT T x is ~A, x^2 is ~A, L is ~A~%
                                        X (SQUARE) L))
   cond of function is (COND
                         ((LISTP L) (FORMAT T ~A is a list~% L))
                         (T NIL))

Now try running the modified function with the default parameter
x is 2, x^2 is 4, L is NIL
NIL is a list

Now try running the modified function with list (1 2 3)
x is 2, x^2 is 4, L is (1 2 3)
(1 2 3) is a list

---Done---