diff --git a/README.md b/README.md
index 586d507..31adb5a 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@
Listing names of all authors ordered by age:
```twig
-{% for author in articles|map(=> _.author)|unique|sort_by(=> _.age) %}
+{% for author in articles|map(=> _.author)|unique_by('===')|sort_by(=> _.age) %}
* {{ author.name }}, {{ author.age }}
{% endfor %}
```
@@ -111,11 +111,29 @@ Returns array of elements that passes a test specified by lambda.
----------------------------------------------------------------
-
-### |unique
-**Signature:** `array|unique`
+
+### |unique_by
+**Signature:** `array|unique_by(lambda|'==='|'==')`
+
+Returns array of unique elements. Uniqueness is checked with
+passed lambda. PHP operators == or === will be used if
+string '==' or '===' is passed instead of lambda.
+
+Lambda should have two arguments - items to check. Keys of
+elements are also passed as third and fourth argument.
+
+```twig
+ {% for i in [1, 2, 2, 3, 1]|unique_by((i1;i2) => i1 == i2) %}
+ {{ i }} {# prints '1 2 3' #}
+ {% endfor %}
+```
+equivalent
+```twig
+ {% for i in [1, 2, 2, 3, 1]|unique_by('==') %}
+ {{ i }} {# prints '1 2 3' #}
+ {% endfor %}
+```
-Returns array with every element occurring only once.
----------------------------------------------------------------