diff --git a/source/engine/index.js b/source/engine/index.js
index 8a78045d7..b1846f54f 100644
--- a/source/engine/index.js
+++ b/source/engine/index.js
@@ -1,8 +1,14 @@
 // This file exports the functions of the public computing library
 import { analyseMany, parseAll } from './traverse.js'
-import { rulesFr, collectDefaults, nestedSituationToPathMap } from './rules'
+import {
+	rulesFr,
+	collectDefaults,
+	nestedSituationToPathMap,
+	enrichRule
+} from './rules'
+import yaml from 'js-yaml'
 
-let parsedRules = parseAll(rulesFr)
+let defaultRules = parseAll(rulesFr)
 
 // The public evaluation function takes a nested object of input values
 let nestedSituationToStateSelector = nestedSituation => dottedName =>
@@ -12,9 +18,13 @@ let nestedSituationToStateSelector = nestedSituation => dottedName =>
 	}[dottedName])
 
 export default {
-	eval: (targetNames, nestedSituation) =>
-		analyseMany(parsedRules, targetNames)(
+	evaluate: (targetNames, nestedSituation, rulesYaml) => {
+		let rules = rulesYaml
+			? (rules = parseAll(yaml.safeLoad(rulesYaml).map(enrichRule)))
+			: defaultRules
+
+		return analyseMany(rules, targetNames)(
 			nestedSituationToStateSelector(nestedSituation)
-		),
-	a: () => 7
+		)
+	}
 }
diff --git a/test/library.test.js b/test/library.test.js
index 02fee93df..ddf90bc55 100644
--- a/test/library.test.js
+++ b/test/library.test.js
@@ -1,11 +1,13 @@
 import { expect } from 'chai'
-import evaluate from '../source/engine/index'
+import Syso from '../source/engine/index'
 import { propEq } from 'ramda'
+import dedent from 'dedent-js'
+console.log(Syso)
 
 describe('library', function() {
 	it('should evaluate one target with no input data', function() {
 		let target = 'contrat salarié . salaire . net'
-		let evaluated = evaluate([target], {
+		let evaluated = Syso.evaluate([target], {
 			'contrat salarié': { salaire: { 'brut de base': 2300 } }
 		})
 		let evaluatedTarget = evaluated.targets.find(propEq('dottedName', target))
@@ -13,4 +15,17 @@ describe('library', function() {
 			.to.have.property('nodeValue')
 			.to.be.within(1779, 1780)
 	})
+
+	it('should let the user replace the default rules', function() {
+		let newRules = `
+- nom: yo
+  formule: 90890
+- nom: ya
+  formule:  yo + 1
+`
+
+		let evaluated = Syso.evaluate(['ya'], {}, newRules)
+
+		expect(evaluated.targets[0].nodeValue).to.equal(90891)
+	})
 })