Generate a defmacro form that transforms code like this:
(define-generic generic ((a a) (b b)) (+ a b))

Pattern to recognize: Macro call that should be transformed by define-generic

Macro definition:
(defmacro define-generic (name (&rest arglist) &body body)
  "Convenience macro to define generics as if they were methods.
ARGLIST can be a method arglist, in which case it's parsed into
generic function arglist by removing method-specific args.

BODY can be a list of `defgeneric' options (processed as-is, even
:method ones) intermixed with other Lisp forms:
- If there's a docstring as the first form, it's processed into
  :documentation option of `defgeneric'.
- If there are non-option forms, they are put together into a separate
  :method option under ARGLIST.
- If there's a `declare' form:
  - If it's generic-friendly (optimize for speed or space), then it's
    put into both generic body and :method body (if any).
  - Otherwise it's only put into :method (creating it if necessary).
- If there's an `:export-generic-name-p' boolean option, export (T) or
  don't export (NIL) the generic symbol.

Example:

\(define-generic add ((a integer) (b integer) &key coerce-to-fixnum &allow-other-keys)
  \"Adds A and B, coercing them to fixnum if the sum is too big.\"
  (if coerce-to-fixnum
      (coerce (+ a b) 'fixnum)
      (+ a b))
  (:method ((a string) (b integer))
   (error \"Cannot use `add' on strings!\")))
=>
\(defgeneric add (a b &key &allow-other-keys)
  (:method ((a integer) (b integer) &key coerce-to-fixnum &allow-other-keys)
   (if coerce-to-fixnum
      (coerce (+ a b) 'fixnum)
      (+ a b)))
  (:method ((a string) (b integer))
   (error \"Cannot use `add' on strings!\"))
  (:documentation \"Adds A and B, coercing them to fixnum if the sum is too big.\"))"
  (multiple-value-bind (body declarations documentation)
      (uiop:parse-body body :documentation t)
    (when (and (= 1 (length body))
               (stringp (first body)))
      (style-warn "Body of DEFINE-GENERIC contains a single string:
~s.
It's interpreted as a method body by default.

If you meant this string to be a generic docstring, and not a method
body, add an explicit NIL or some other body form:
(define-generic name (arg1 arg2)
  \"Docstring\"
  'body-form)

If you meant it as a body form, do nothing, or provide explicit
documentation or any other option:
(define-generic name (arg1 arg2)
  \"Docstring\"
  (:documentation \"Real docstring\"))" (first body)))
    (let* ((generic-declarations (remove-if (lambda (d)
                                              (or (> (length d) 2)
                                                  (not (eq 'optimize (first (second d))))
                                                  (notevery (lambda (o)
                                                              (let ((feature (first (uiop:ensure-list o))))
                                                                (or (eq feature 'speed)
                                                                    (eq feature 'space))))
                                                            (rest (second d)))))
                                            declarations))
           (options (remove-if-not (lambda (form)
                                     (keywordp (first (uiop:ensure-list form))))
                                   body))
           (method-body (remove-if (lambda (f) (member f options :test #'equal)) body))
           (export-generic-name-p (find :export-generic-name-p options :key #'first))
           (options (remove export-generic-name-p options :test #'equal))
           (export-p (if export-generic-name-p
                         (second export-generic-name-p)
                         *export-generic-name-p*))
           (generalized-arglist (generalize-arglist arglist)))
      (when (and (not (equal generalized-arglist arglist))
                 (every #'null (list method-body options)))
        (style-warn "Specialized arglist used without method body in define-generic: ~a" arglist))
      `(prog1
           (defgeneric ,name ,generalized-arglist
             ,@generic-declarations
             ,@(when (or method-body
                         (not (equal generic-declarations
                                     declarations)))
                 `((:method ,arglist
                     ,@declarations
                     ,@method-body)))
             ,@options
             ,@(when documentation
                 `((:documentation ,documentation))))
         ;; FIXME: Maybe only export if package of generic function name
         ;; matches *PACKAGE*?
         ,@(when export-p
             `((eval-when (:compile-toplevel :load-toplevel :execute)
                 (export ',name ,(package-name *package*)))))
         ,@(let ((documentation (or documentation
                                    (second (first (member :documentation options :key #'first))))))
             (when documentation
               `((setf (documentation ',name 'function) ,documentation)
                 (setf (documentation (fdefinition ',name) 'function) ,documentation))))))))

Macroexpansion:
(prog1 (defgeneric generic (a b) (:method ((a a) (b b)) (+ a b))))