(define-syntax CaseMettaHelper
  (syntax-rules (else)
    ((_ else bi) (list else bi))
    ((_ ai bi) (list (list ai) bi))))

(define-syntax CaseMettaClean
  (syntax-rules (else)
    ((_ var ((a1 else)))
     (case var (else b1)))
    ((_ var ((a1 b1)))
     (case var ((a1) b1)))
    ((_ var ((ai bi) ...))
     (list var (CaseMettaHelper ai bi) ...))))
     ;^^^^
     ;expected would have been to be able to use directly
     ;case instead of list here, but case is so greedy that
     ;CaseMettaHelper won't be evaluated before it complains about it.

(define-syntax CaseMetta
  (syntax-rules (else)
    ((_ var cases)
     (eval (let ((x (CaseMettaClean var cases))) (append (list 'case) x))))))
     ;forces to have the CaseMettaClean evaluated before filling in "case".

(display (CaseMetta 3 ((1 2) (3 4))))
;4