diff --git a/jolt-core/jolt/backend_scheme.clj b/jolt-core/jolt/backend_scheme.clj
index 188e169..4f5067c 100644
--- a/jolt-core/jolt/backend_scheme.clj
+++ b/jolt-core/jolt/backend_scheme.clj
@@ -80,6 +80,10 @@
 ;; :num-kind :double|:long when every operand is that kind; these are the Chez
 ;; flonum/fixnum ops it lowers to — no generic dispatch, fixnums unboxed. fl?/fx?
 ;; comparisons carry the question mark; fl+/fx+ don't.
+;;
+;; CONTRACT: every op name jolt.passes.numeric/dbl-spec (resp. lng-spec) tags must
+;; have an entry here, or emit-numeric splices a nil op string into the output. Keep
+;; these tables and those specializers in sync.
 (def ^:private dbl-ops
   {"+" "fl+" "-" "fl-" "*" "fl*" "/" "fl/" "min" "flmin" "max" "flmax"
    "<" "fl<?" ">" "fl>?" "<=" "fl<=?" ">=" "fl>=?" "=" "fl=?" "==" "fl=?"})
diff --git a/jolt-core/jolt/passes/numeric.clj b/jolt-core/jolt/passes/numeric.clj
index 9f6e732..0bd9033 100644
--- a/jolt-core/jolt/passes/numeric.clj
+++ b/jolt-core/jolt/passes/numeric.clj
@@ -28,6 +28,8 @@
 
 ;; result kind of a double-specialized op at this name/arity, or nil if N/A.
 ;; arithmetic -> :double; comparison -> :bool (operands specialized, result not numeric).
+;; Every op name dbl-spec / lng-spec returns non-nil for must have a Chez op in
+;; jolt.backend-scheme/dbl-ops resp. lng-ops, or emit-numeric splices a nil op.
 (defn- dbl-spec [nm n]
   (cond
     (and (>= n 1) (contains? #{"+" "-" "*" "/" "min" "max"} nm)) :double