(defun common-2d-vector-p (S)
"Takes an S-expression \(list\) and returns T if it is a two-elemented single-level list of floats and NIL otherwise."
    (cond
        ((atom S) nil) ; Seeing an atom (or a plain NIL)? Not a 2d-vector.
        ((null (car S)) nil) ; Non-empty list containing NIL as its CAR? Not a 2d-vector.
        ((atom (cdr S)) nil) ; NEL containing atomic CDR? Not a 2d-vector - more like 1d-vector or a plain dp.
        (T (cond             ; CAR and CDR are both not NIL, good! Seeking for atomic CAR & CADR next.
            ((and (atom (car S)) (atom (cadr S))) (cond
                                                    ((and (floatp (car S)) (floatp (cadr S))) T)
                                                    (T nil)))
            (T nil)))))