Guards

[gordon-quad]

Something like that

(match x
(((a b c) (eq a b)) (do something when a equals to b))
(((a b c) (eq a outervar)) (do something else when a equals to outervar))
((x t) (do another thing else)))

[gordon-quad]

or perhaps optional "when" will be much more interesting
(match x (((a b c) (:when (eq a b)) (do something))

[naryl]

(match x
  ((a b c)
   (:when (eq a b))
   (do something with a b and c)))