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index 370db08..9c58d69 100644
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index 5bddb16..1bb2738 100644
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@@ -424,44 +422,44 @@
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@@ -487,7 +485,7 @@
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@@ -582,16 +580,16 @@
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@@ -627,20 +625,20 @@
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@@ -676,57 +674,56 @@
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+                "sebastian/resource-operations": "^2.0.1",
+                "sebastian/type": "^1.1.3",
                 "sebastian/version": "^2.0.1"
             },
-            "conflict": {
-                "phpunit/phpunit-mock-objects": "*"
-            },
             "require-dev": {
                 "ext-pdo": "*"
             },
             "suggest": {
                 "ext-soap": "*",
                 "ext-xdebug": "*",
-                "phpunit/php-invoker": "^2.0"
+                "phpunit/php-invoker": "^2.0.0"
             },
             "bin": [
                 "phpunit"
@@ -734,7 +731,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "7.5-dev"
+                    "dev-master": "8.5-dev"
                 }
             },
             "autoload": {
@@ -760,7 +757,7 @@
                 "testing",
                 "xunit"
             ],
-            "time": "2018-12-12T07:20:32+00:00"
+            "time": "2020-05-22T13:51:52+00:00"
         },
         {
             "name": "sebastian/code-unit-reverse-lookup",
@@ -873,23 +870,23 @@
         },
         {
             "name": "sebastian/diff",
-            "version": "3.0.1",
+            "version": "3.0.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/diff.git",
-                "reference": "366541b989927187c4ca70490a35615d3fef2dce"
+                "reference": "720fcc7e9b5cf384ea68d9d930d480907a0c1a29"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/diff/zipball/366541b989927187c4ca70490a35615d3fef2dce",
-                "reference": "366541b989927187c4ca70490a35615d3fef2dce",
+                "url": "https://api.github.com/repos/sebastianbergmann/diff/zipball/720fcc7e9b5cf384ea68d9d930d480907a0c1a29",
+                "reference": "720fcc7e9b5cf384ea68d9d930d480907a0c1a29",
                 "shasum": ""
             },
             "require": {
                 "php": "^7.1"
             },
             "require-dev": {
-                "phpunit/phpunit": "^7.0",
+                "phpunit/phpunit": "^7.5 || ^8.0",
                 "symfony/process": "^2 || ^3.3 || ^4"
             },
             "type": "library",
@@ -925,32 +922,35 @@
                 "unidiff",
                 "unified diff"
             ],
-            "time": "2018-06-10T07:54:39+00:00"
+            "time": "2019-02-04T06:01:07+00:00"
         },
         {
             "name": "sebastian/environment",
-            "version": "4.0.1",
+            "version": "4.2.3",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/environment.git",
-                "reference": "febd209a219cea7b56ad799b30ebbea34b71eb8f"
+                "reference": "464c90d7bdf5ad4e8a6aea15c091fec0603d4368"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/environment/zipball/febd209a219cea7b56ad799b30ebbea34b71eb8f",
-                "reference": "febd209a219cea7b56ad799b30ebbea34b71eb8f",
+                "url": "https://api.github.com/repos/sebastianbergmann/environment/zipball/464c90d7bdf5ad4e8a6aea15c091fec0603d4368",
+                "reference": "464c90d7bdf5ad4e8a6aea15c091fec0603d4368",
                 "shasum": ""
             },
             "require": {
                 "php": "^7.1"
             },
             "require-dev": {
-                "phpunit/phpunit": "^7.4"
+                "phpunit/phpunit": "^7.5"
+            },
+            "suggest": {
+                "ext-posix": "*"
             },
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "4.0-dev"
+                    "dev-master": "4.2-dev"
                 }
             },
             "autoload": {
@@ -975,20 +975,20 @@
                 "environment",
                 "hhvm"
             ],
-            "time": "2018-11-25T09:31:21+00:00"
+            "time": "2019-11-20T08:46:58+00:00"
         },
         {
             "name": "sebastian/exporter",
-            "version": "3.1.0",
+            "version": "3.1.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/exporter.git",
-                "reference": "234199f4528de6d12aaa58b612e98f7d36adb937"
+                "reference": "68609e1261d215ea5b21b7987539cbfbe156ec3e"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/exporter/zipball/234199f4528de6d12aaa58b612e98f7d36adb937",
-                "reference": "234199f4528de6d12aaa58b612e98f7d36adb937",
+                "url": "https://api.github.com/repos/sebastianbergmann/exporter/zipball/68609e1261d215ea5b21b7987539cbfbe156ec3e",
+                "reference": "68609e1261d215ea5b21b7987539cbfbe156ec3e",
                 "shasum": ""
             },
             "require": {
@@ -1015,6 +1015,10 @@
                 "BSD-3-Clause"
             ],
             "authors": [
+                {
+                    "name": "Sebastian Bergmann",
+                    "email": "sebastian@phpunit.de"
+                },
                 {
                     "name": "Jeff Welch",
                     "email": "whatthejeff@gmail.com"
@@ -1023,17 +1027,13 @@
                     "name": "Volker Dusch",
                     "email": "github@wallbash.com"
                 },
-                {
-                    "name": "Bernhard Schussek",
-                    "email": "bschussek@2bepublished.at"
-                },
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                },
                 {
                     "name": "Adam Harvey",
                     "email": "aharvey@php.net"
+                },
+                {
+                    "name": "Bernhard Schussek",
+                    "email": "bschussek@gmail.com"
                 }
             ],
             "description": "Provides the functionality to export PHP variables for visualization",
@@ -1042,27 +1042,30 @@
                 "export",
                 "exporter"
             ],
-            "time": "2017-04-03T13:19:02+00:00"
+            "time": "2019-09-14T09:02:43+00:00"
         },
         {
             "name": "sebastian/global-state",
-            "version": "2.0.0",
+            "version": "3.0.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/global-state.git",
-                "reference": "e8ba02eed7bbbb9e59e43dedd3dddeff4a56b0c4"
+                "reference": "edf8a461cf1d4005f19fb0b6b8b95a9f7fa0adc4"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/global-state/zipball/e8ba02eed7bbbb9e59e43dedd3dddeff4a56b0c4",
-                "reference": "e8ba02eed7bbbb9e59e43dedd3dddeff4a56b0c4",
+                "url": "https://api.github.com/repos/sebastianbergmann/global-state/zipball/edf8a461cf1d4005f19fb0b6b8b95a9f7fa0adc4",
+                "reference": "edf8a461cf1d4005f19fb0b6b8b95a9f7fa0adc4",
                 "shasum": ""
             },
             "require": {
-                "php": "^7.0"
+                "php": "^7.2",
+                "sebastian/object-reflector": "^1.1.1",
+                "sebastian/recursion-context": "^3.0"
             },
             "require-dev": {
-                "phpunit/phpunit": "^6.0"
+                "ext-dom": "*",
+                "phpunit/phpunit": "^8.0"
             },
             "suggest": {
                 "ext-uopz": "*"
@@ -1070,7 +1073,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "2.0-dev"
+                    "dev-master": "3.0-dev"
                 }
             },
             "autoload": {
@@ -1093,7 +1096,7 @@
             "keywords": [
                 "global state"
             ],
-            "time": "2017-04-27T15:39:26+00:00"
+            "time": "2019-02-01T05:30:01+00:00"
         },
         {
             "name": "sebastian/object-enumerator",
@@ -1282,6 +1285,52 @@
             "homepage": "https://www.github.com/sebastianbergmann/resource-operations",
             "time": "2018-10-04T04:07:39+00:00"
         },
+        {
+            "name": "sebastian/type",
+            "version": "1.1.3",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/sebastianbergmann/type.git",
+                "reference": "3aaaa15fa71d27650d62a948be022fe3b48541a3"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/sebastianbergmann/type/zipball/3aaaa15fa71d27650d62a948be022fe3b48541a3",
+                "reference": "3aaaa15fa71d27650d62a948be022fe3b48541a3",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.2"
+            },
+            "require-dev": {
+                "phpunit/phpunit": "^8.2"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "1.1-dev"
+                }
+            },
+            "autoload": {
+                "classmap": [
+                    "src/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "BSD-3-Clause"
+            ],
+            "authors": [
+                {
+                    "name": "Sebastian Bergmann",
+                    "email": "sebastian@phpunit.de",
+                    "role": "lead"
+                }
+            ],
+            "description": "Collection of value objects that represent the types of the PHP type system",
+            "homepage": "https://github.com/sebastianbergmann/type",
+            "time": "2019-07-02T08:10:15+00:00"
+        },
         {
             "name": "sebastian/version",
             "version": "2.0.1",
@@ -1327,16 +1376,16 @@
         },
         {
             "name": "squizlabs/php_codesniffer",
-            "version": "3.4.0",
+            "version": "3.5.5",
             "source": {
                 "type": "git",
                 "url": "https://github.com/squizlabs/PHP_CodeSniffer.git",
-                "reference": "379deb987e26c7cd103a7b387aea178baec96e48"
+                "reference": "73e2e7f57d958e7228fce50dc0c61f58f017f9f6"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/squizlabs/PHP_CodeSniffer/zipball/379deb987e26c7cd103a7b387aea178baec96e48",
-                "reference": "379deb987e26c7cd103a7b387aea178baec96e48",
+                "url": "https://api.github.com/repos/squizlabs/PHP_CodeSniffer/zipball/73e2e7f57d958e7228fce50dc0c61f58f017f9f6",
+                "reference": "73e2e7f57d958e7228fce50dc0c61f58f017f9f6",
                 "shasum": ""
             },
             "require": {
@@ -1369,25 +1418,25 @@
                 }
             ],
             "description": "PHP_CodeSniffer tokenizes PHP, JavaScript and CSS files and detects violations of a defined set of coding standards.",
-            "homepage": "http://www.squizlabs.com/php-codesniffer",
+            "homepage": "https://github.com/squizlabs/PHP_CodeSniffer",
             "keywords": [
                 "phpcs",
                 "standards"
             ],
-            "time": "2018-12-19T23:57:18+00:00"
+            "time": "2020-04-17T01:09:41+00:00"
         },
         {
             "name": "symfony/polyfill-ctype",
-            "version": "v1.10.0",
+            "version": "v1.17.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/symfony/polyfill-ctype.git",
-                "reference": "e3d826245268269cd66f8326bd8bc066687b4a19"
+                "reference": "e94c8b1bbe2bc77507a1056cdb06451c75b427f9"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/symfony/polyfill-ctype/zipball/e3d826245268269cd66f8326bd8bc066687b4a19",
-                "reference": "e3d826245268269cd66f8326bd8bc066687b4a19",
+                "url": "https://api.github.com/repos/symfony/polyfill-ctype/zipball/e94c8b1bbe2bc77507a1056cdb06451c75b427f9",
+                "reference": "e94c8b1bbe2bc77507a1056cdb06451c75b427f9",
                 "shasum": ""
             },
             "require": {
@@ -1399,7 +1448,7 @@
             "type": "library",
             "extra": {
                 "branch-alias": {
-                    "dev-master": "1.9-dev"
+                    "dev-master": "1.17-dev"
                 }
             },
             "autoload": {
@@ -1415,13 +1464,13 @@
                 "MIT"
             ],
             "authors": [
-                {
-                    "name": "Symfony Community",
-                    "homepage": "https://symfony.com/contributors"
-                },
                 {
                     "name": "Gert de Pagter",
                     "email": "BackEndTea@gmail.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
                 }
             ],
             "description": "Symfony polyfill for ctype functions",
@@ -1432,20 +1481,20 @@
                 "polyfill",
                 "portable"
             ],
-            "time": "2018-08-06T14:22:27+00:00"
+            "time": "2020-05-12T16:14:59+00:00"
         },
         {
             "name": "theseer/tokenizer",
-            "version": "1.1.0",
+            "version": "1.1.3",
             "source": {
                 "type": "git",
                 "url": "https://github.com/theseer/tokenizer.git",
-                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b"
+                "reference": "11336f6f84e16a720dae9d8e6ed5019efa85a0f9"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/theseer/tokenizer/zipball/cb2f008f3f05af2893a87208fe6a6c4985483f8b",
-                "reference": "cb2f008f3f05af2893a87208fe6a6c4985483f8b",
+                "url": "https://api.github.com/repos/theseer/tokenizer/zipball/11336f6f84e16a720dae9d8e6ed5019efa85a0f9",
+                "reference": "11336f6f84e16a720dae9d8e6ed5019efa85a0f9",
                 "shasum": ""
             },
             "require": {
@@ -1472,36 +1521,33 @@
                 }
             ],
             "description": "A small library for converting tokenized PHP source code into XML and potentially other formats",
-            "time": "2017-04-07T12:08:54+00:00"
+            "time": "2019-06-13T22:48:21+00:00"
         },
         {
             "name": "webmozart/assert",
-            "version": "1.4.0",
+            "version": "1.8.0",
             "source": {
                 "type": "git",
                 "url": "https://github.com/webmozart/assert.git",
-                "reference": "83e253c8e0be5b0257b881e1827274667c5c17a9"
+                "reference": "ab2cb0b3b559010b75981b1bdce728da3ee90ad6"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/webmozart/assert/zipball/83e253c8e0be5b0257b881e1827274667c5c17a9",
-                "reference": "83e253c8e0be5b0257b881e1827274667c5c17a9",
+                "url": "https://api.github.com/repos/webmozart/assert/zipball/ab2cb0b3b559010b75981b1bdce728da3ee90ad6",
+                "reference": "ab2cb0b3b559010b75981b1bdce728da3ee90ad6",
                 "shasum": ""
             },
             "require": {
                 "php": "^5.3.3 || ^7.0",
                 "symfony/polyfill-ctype": "^1.8"
             },
+            "conflict": {
+                "vimeo/psalm": "<3.9.1"
+            },
             "require-dev": {
-                "phpunit/phpunit": "^4.6",
-                "sebastian/version": "^1.0.1"
+                "phpunit/phpunit": "^4.8.36 || ^7.5.13"
             },
             "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.3-dev"
-                }
-            },
             "autoload": {
                 "psr-4": {
                     "Webmozart\\Assert\\": "src/"
@@ -1523,7 +1569,7 @@
                 "check",
                 "validate"
             ],
-            "time": "2018-12-25T11:19:39+00:00"
+            "time": "2020-04-18T12:12:48+00:00"
         }
     ],
     "aliases": [],
@@ -1532,7 +1578,7 @@
     "prefer-stable": false,
     "prefer-lowest": false,
     "platform": {
-        "php": ">=5.3.0"
+        "php": ">=7.2.0"
     },
     "platform-dev": []
 }
diff --git a/src/Lists.php b/src/Lists.php
deleted file mode 100644
index 51c9d7a..0000000
--- a/src/Lists.php
+++ /dev/null
@@ -1,131 +0,0 @@
-<?php
-
-namespace PhpPairs\Lists;
-
-use function PhpPairs\Pairs\cons;
-use function PhpPairs\Pairs\car;
-use function PhpPairs\Pairs\cdr;
-use function PhpPairs\Pairs\toString;
-
-/**
- * Creates new list with given $elements
- * @param mixed[] $elements elements to add
- * @return callable list
- */
-function l(...$elements)
-{
-    return array_reduce(array_reverse($elements), function ($acc, $item) {
-        return cons($item, $acc);
-    });
-}
-
-/**
- * Applies callable function $func to list $list
- * @param  callable $list list
- * @param  callable $func function
- * @return callable list
- */
-function map($list, callable $func)
-{
-    $map = function ($items, $acc) use (&$map, $func) {
-        if (is_null($items)) {
-            return reverse($acc);
-        }
-        return $map(cdr($items), cons($func(car($items)), $acc));
-    };
-
-    return $map($list, null);
-}
-
-/**
- * Filters list $list using callable function $func
- * @param  callable $list list
- * @param  callable $func function
- * @return callable list
- */
-function filter($list, callable $func)
-{
-    $map = function ($func, $items) use (&$map) {
-        if ($items === null) {
-            return null;
-        } else {
-            $curr = car($items);
-            $rest = $map($func, cdr($items));
-            // filter
-            return $func($curr) ? cons($curr, $rest) : $rest;
-        }
-    };
-
-    return $map($func, $list);
-}
-
-/**
- * Collapses the list $list using callable function $func
- * @param  callable $list list
- * @param  callable $func function
- * @param  mixed   $acc
- * @return mixed
- */
-function accumulate($list, callable $func, $acc = null)
-{
-    return reduce($list, $func, $acc);
-}
-
-/**
- * Collapses the list $list using callable function $func
- * @param  callable $list list
- * @param  callable $func function
- * @param  mixed   $acc
- * @return mixed
- */
-function reduce($list, callable $func, $acc = null)
-{
-    $iter = function ($items, $acc) use (&$iter, $func) {
-        return is_null($items) ? $acc : $iter(cdr($items), $func(car($items), $acc));
-    };
-
-    return $iter($list, $acc);
-}
-
-/**
- * Concatenates two lists
- * @param  pair $list1
- * @param  pair $list2
- * @return callable new list
- */
-function append($list1, $list2)
-{
-    if ($list1 === null) {
-        return $list2;
-    } else {
-        return cons(car($list1), append(cdr($list1), $list2));
-    }
-}
-
-/**
- * Reverse list $list
- * @param  callable $list list
- * @return callable result
- */
-function reverse($list)
-{
-    $iter = function ($items, $acc) use (&$iter) {
-        return is_null($items) ? $acc : $iter(cdr($items), cons(car($items), $acc));
-    };
-
-    return $iter($list, null);
-}
-
-/**
- * Returns length of list
- * @param  callable $list list
- * @return integer  length
- */
-function length($list)
-{
-    if ($list === null || !is_callable($list)) {
-        return 0;
-    } else {
-        return 1 + length(cdr($list));
-    }
-}
diff --git a/src/Pairs.php b/src/Pairs.php
index 67c0f92..c144fdd 100644
--- a/src/Pairs.php
+++ b/src/Pairs.php
@@ -4,18 +4,22 @@
 
 /**
  * Creates a pair with two values
- * @param mixed $x first value
- * @param mixed $y second value
+ * @param mixed $a first value
+ * @param mixed $b second value
  * @return callable pair
+ * @example
+ * $pair = cons(5, 'hello');
+ * @example
+ * $pair = cons(cons(1, null), 'world');
  */
-function cons($x, $y)
+function cons($a, $b)
 {
-    return function ($method) use ($x, $y) {
+    return function ($method) use ($a, $b) {
         switch ($method) {
             case "car":
-                return $x;
+                return $a;
             case "cdr":
-                return $y;
+                return $b;
             default:
                 throw new \InvalidArgumentException("Invalid method $method.");
         }
@@ -23,53 +27,76 @@ function cons($x, $y)
 }
 
 /**
- * Extracts first value from pair
- * @param callalble $pair
+ * Check if something is pair
+ * @param callable $pair
+ * @return boolean
+ * @example
+ * $pair = cons(5, 'hello');
+ * isPair($pair); // true
+ * isPair(5); // false
+ */
+function isPair($pair)
+{
+    return is_callable($pair);
+}
+
+function checkPair($pair)
+{
+    if (!isPair($pair)) {
+        $value = is_array($pair) ? 'array' : (string) $pair;
+        throw new \Exception("Argument must be pair, but it was '{$value}'");
+    }
+}
+
+/**
+ * Get car (first element) from pair
+ * @param callable $pair
  * @return mixed
  */
 function car(callable $pair)
 {
+    checkPair($pair);
     return $pair("car");
 }
 
 /**
- * Extracts second value from pair
- * @param callalble $pair
+ * Get cdr (second element) from pair
+ * @param callable $pair
  * @return mixed
  */
 function cdr(callable $pair)
 {
+    checkPair($pair);
     return $pair("cdr");
 }
 
 /**
- * Converts given list to string
- * @param callalble $list
+ * Convert pair to string (recursively)
+ * @param callable $pair
  * @return string
+ * @example
+ * toString(cons('', 10)); // ('', 10)
  */
-function toString($list)
+function toString($pair)
 {
-    if (!isPair($list)) {
-        return $list;
-    }
+    checkPair($pair);
+    $rec = function ($p) use (&$rec) {
+        if (!isPair($p)) {
+            if ($p === null) {
+                return 'null';
+            }
+            if ($p === '') {
+                return "''";
+            }
 
-    $iter = function ($items, array $acc = []) use (&$iter) {
-        if ($items == null) {
-            return $acc;
+            return (string) $p;
         }
-        return $iter(cdr($items), array_merge($acc, [toString(car($items))]));
-    };
-    $arr = $iter($list);
 
-    return "(" . implode(", ", $arr) . ")";
-}
+        $recLeft = $rec(car($p));
+        $recRight = $rec(cdr($p));
 
-/**
- * Checks whether given $pair is valid pair
- * @param callable $pair
- * @return boolean
- */
-function isPair($pair)
-{
-    return is_callable($pair);
+        return "({$recLeft}, {$recRight})";
+    };
+
+    return $rec($pair);
 }
diff --git a/tests/ListsTest.php b/tests/ListsTest.php
deleted file mode 100644
index e15fea0..0000000
--- a/tests/ListsTest.php
+++ /dev/null
@@ -1,95 +0,0 @@
-<?php
-
-namespace PhpPairs\tests;
-
-use function PhpPairs\Pairs\cons;
-use function PhpPairs\Pairs\car;
-use function PhpPairs\Pairs\cdr;
-use function PhpPairs\Pairs\toString;
-use function PhpPairs\Lists\length;
-use function PhpPairs\Lists\reverse;
-use function PhpPairs\Lists\l;
-use function PhpPairs\Lists\map;
-use function PhpPairs\Lists\filter;
-use function PhpPairs\Lists\reduce;
-
-use PHPUnit\Framework\TestCase;
-
-class ListsTest extends TestCase
-{
-
-    public function testL()
-    {
-        $this->assertEquals(toString(l()), toString(l()));
-        $list = cons(1, cons((cons(3, cons(4, null))), cons(5, null)));
-        $this->assertEquals(toString($list), toString(l(1, l(3, 4), 5)));
-    }
-
-    public function testLength()
-    {
-        $this->assertEquals(0, length(l()));
-        $list = l(1, 2, 3);
-        $this->assertEquals(3, length($list));
-    }
-
-    public function testReverse()
-    {
-        $this->assertEquals(toString(l()), toString(reverse(l())));
-
-        $list = l(1, 2, 3);
-        $expected = toString(l(3, 2, 1));
-        $this->assertEquals($expected, toString(reverse($list)));
-    }
-
-    public function testMap()
-    {
-        $list = l(1, 2, 3);
-        $expected = toString(l(3, 4, 5));
-        $map = map($list, function ($x) {
-            return $x + 2;
-        });
-        $this->assertEquals($expected, toString($map));
-
-        $list2 = l();
-        $map = map($list2, function ($x) {
-            return $x + 2;
-        });
-        $this->assertEquals(toString(l()), toString($map));
-    }
-
-    public function testFilter()
-    {
-        $list = l(2, 3, 4);
-        $expected = toString(l(2, 4));
-        $filter = filter($list, function ($x) {
-            return $x % 2 == 0;
-        });
-
-        $this->assertEquals(2, length($filter));
-        $this->assertEquals($expected, toString($filter));
-
-        $list2 = l();
-        $expected = toString(l());
-        $filtered = filter($list2, function ($x) {
-            return $x % 2 == 0;
-        });
-        $this->assertEquals($expected, toString($filtered));
-    }
-
-    public function testReduce()
-    {
-        $list = l(1, 2, 3);
-        $expected = 6;
-        $reduced = reduce($list, function ($x, $acc) {
-            return $x + $acc;
-        }, 0);
-        $this->assertEquals($expected, $reduced);
-
-        $list2 = l();
-        $expected = null;
-        $reduced2 = reduce($list2, function ($x, $acc) {
-            return $x + $acc;
-        });
-        $this->assertEquals($expected, $reduced2);
-    }
-}
diff --git a/tests/PairsTest.php b/tests/PairsTest.php
index 78c4de3..b4c0d66 100644
--- a/tests/PairsTest.php
+++ b/tests/PairsTest.php
@@ -2,18 +2,46 @@
 
 namespace PhpPairs\Pairs\tests;
 
+use PHPUnit\Framework\TestCase;
+
 use function PhpPairs\Pairs\cons;
 use function PhpPairs\Pairs\car;
 use function PhpPairs\Pairs\cdr;
-
-use PHPUnit\Framework\TestCase;
+use function PhpPairs\Pairs\toString;
+use function PhpPairs\Pairs\checkPair;
 
 class PairsTest extends TestCase
 {
     public function testPairs()
     {
-        $pair = cons(1, 2);
-        $this->assertEquals(1, car($pair));
-        $this->assertEquals(2, cdr($pair));
+        $pair = cons(3, 4);
+
+        $this->assertEquals(3, car($pair));
+        $this->assertEquals(4, cdr($pair));
+    }
+
+    public function testToString()
+    {
+        $pair1 = cons(3, cons(3, 2));
+        $pair2 = cons(cons(3, 5), cons(1, null));
+        $pair3 = cons(10, -10);
+        $pair4 = cons('', -10);
+
+        $this->assertEquals('(3, (3, 2))', toString($pair1));
+        $this->assertEquals('((3, 5), (1, null))', toString($pair2));
+        $this->assertEquals('(10, -10)', toString($pair3));
+        $this->assertEquals("('', -10)", toString($pair4));
+    }
+
+    public function testCheckPair()
+    {
+        $this->expectExceptionMessage("Argument must be pair, but it was '1'");
+        checkPair(1);
+        $this->expectExceptionMessage("Argument must be pair, but it was 'some string'");
+        checkPair('some string');
+        $this->expectExceptionMessage("Argument must be pair, but it was 'array'");
+        checkPair([1, 2]);
+        $this->expectExceptionMessage("Argument must be pair, but it was 'null'");
+        checkPair(null);
     }
 }