I would just use this first form below. A macro you may be able to adapt for your purposes follows.
(let a2 (+ a c)
(if (> a2 b) (= a a2)))
(mac =if (var expr test)
(w/uniq result
`(let ,result ,expr
(if (,test ,result) (= ,var ,result))
,var)))
(=if a (+ a c) [> _ b])
