diff --git a/pkg/nsrule/env.go b/pkg/nsrule/env.go
index a8ab312..fdd6db3 100644
--- a/pkg/nsrule/env.go
+++ b/pkg/nsrule/env.go
@@ -2,14 +2,18 @@ package nsrule
 
 import (
 	"maps"
+	"reflect"
+
+	"github.com/antonmedv/expr"
 )
 
 // Env contains data used to evaluate rules.
 type Env map[string]any
 
 var (
-	dummyEnv   = Env{}
-	defaultEnv = Env{}
+	dummyEnv     = Env{}
+	defaultEnv   = Env{}
+	extraOptions []expr.Option
 )
 
 // NewEnv shallow-copies the default values into a new Env.
@@ -43,3 +47,11 @@ func Define[T any](name string) func(Env, T) {
 func DefineDefault[T any](name string, def T) func(Env, T) {
 	return define[T](name, def, false)
 }
+
+// DefineOperator overloads an operator.
+func DefineOperator[T any](op string, fn T) string {
+	name := op + " " + reflect.TypeOf(fn).String()
+	extraOptions = append(extraOptions, expr.Operator(op, name))
+	define[T](name, fn, false)
+	return name
+}
diff --git a/pkg/nsrule/rule.go b/pkg/nsrule/rule.go
index 4beac2a..baa6af0 100644
--- a/pkg/nsrule/rule.go
+++ b/pkg/nsrule/rule.go
@@ -171,7 +171,7 @@ func ParseRules(r io.Reader, name string) ([]Rule, error) {
 				name: name,
 				line: expN,
 			}
-			if v, err := expr.Compile(expB.String(), expr.AsBool(), expr.Optimize(true), expr.Env(dummyEnv)); err != nil { // TODO: dummy env
+			if v, err := expr.Compile(expB.String(), append([]expr.Option{expr.AsBool(), expr.Optimize(true), expr.Env(dummyEnv)}, extraOptions...)...); err != nil { // TODO: dummy env
 				return rs, fmt.Errorf("line %d: compile rule expression: %w", expN, err)
 			} else {
 				r.expr = v