replace : {n m : Level} {A : Set n} {t : Set m} → (env : Env A )
    → (next : Env A → t )
    → (exit : Env A → t) → t
replace env next exit = replace-c (varl env) env next exit where
    replace-c : {n m : Level} {A : Set n} {t : Set m} → List (bt A) → (env : Env A )
        → (next : Env A → t )
        → (exit : Env A → t) → t
    replace-c st env next exit with st
    ... | [] = exit env
    ... | leaf ∷ st1 = replace-c st1 env next exit
    ... | n@(node key-t value ltree rtree) ∷ st1 with <-cmp (varn env) (key-t)
    ... | tri< a ¬b ¬c = replace-c st1 record env{vart = (node key-t value (vart env) rtree); varl = st1} next exit
    ... | tri≈ ¬a b ¬c = replace-c st1 record env{vart = (node key-t (varv env) ltree rtree); varl = st1} next exit
    ... | tri> ¬a ¬b c = replace-c st1 record env{vart = (node key-t value ltree (vart env)); varl = st1} next exit