sp--syntax-class-to-char: Symbol’s function definition is void: fixnump

[zahardzhan]

The alias sp--syntax-class-to-char uses the function fixnump, which is available only in Emacs 27 and later.
Consequently, calling backward-kill-word in earlier versions (e.g. Emacs 25) signals:

sp--syntax-class-to-char: Symbol’s function definition is void: fixnump

Providing the missing definition eliminates the error:

(unless (fboundp 'fixnump)
  (defun fixnump (object)
    "Return t if OBJECT is a fixnum."
    (and (integerp object)
         (<= most-negative-fixnum object most-positive-fixnum))))

(unless (fboundp 'defvar-local)
  (defmacro defvar-local (var val &optional docstring)
    "Define VAR as a buffer-local variable with default value VAL.
Like `defvar' but additionally marks the variable as automatically
buffer-local wherever it is set."
    (declare (debug defvar) (doc-string 3))
    ;; Can't use backquote here, it's too early in the bootstrap.
    (list 'progn (list 'defvar var val docstring)
          (list 'make-variable-buffer-local (list 'quote var)))))