Huh, I guess I "knew" since keywords are just regular symbols in the KEYWORD package in this situation, but it never occurred to me to do something that cursed. Very cool!

But this made me realize there's no way in CL (even with PROGV shenanigans) to temporarily bind the function value of a symbol, unlike the variable one through a simple LET =(

So this turns in this horrible (and incorrect, since this doesn't restore the previous FDEFINITIONs) thing when you want to use this hack "properly":

  (defun calculate (instructions)
    (setf (symbol-function :add)      #'+
          (symbol-function :subtract) #'-
          (symbol-function :multiply) #'*)
    (unwind-protect
         (reduce
          (lambda (result op-value)
            (funcall (car op-value) result (cadr op-value)))
          instructions
          :initial-value 0)
      (fmakunbound :add)
      (fmakunbound :subtract)
      (fmakunbound :multiply)))