diff --git a/composer.json b/composer.json
index 42e2e1b..31ac2d0 100644
--- a/composer.json
+++ b/composer.json
@@ -41,7 +41,9 @@
         "phplrt/visitor": "^3.6",
         "phplrt/lexer": "^3.6",
         "phpunit/phpunit": "^9.6|^10.0",
-        "vimeo/psalm": "^5.22",
+        "phpstan/extension-installer": "^1.4",
+        "phpstan/phpstan": "^1.11",
+        "phpstan/phpstan-strict-rules": "^1.6",
         "jetbrains/phpstorm-attributes": "^1.0"
     },
     "autoload-dev": {
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..f913176
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,12 @@
+includes:
+  - phar://phpstan.phar/conf/bleedingEdge.neon
+parameters:
+  level: 1
+  strictRules:
+    allRules: true
+  fileExtensions:
+    - php
+  paths:
+    - src
+  tmpDir: vendor/.cache.phpstan
+  reportUnmatchedIgnoredErrors: false
diff --git a/psalm.xml b/psalm.xml
deleted file mode 100644
index 7d47ed8..0000000
--- a/psalm.xml
+++ /dev/null
@@ -1,16 +0,0 @@
-<?xml version="1.0"?>
-<psalm
-    errorLevel="4"
-    resolveFromConfigFile="true"
-    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xmlns="https://getpsalm.org/schema/config"
-    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
->
-    <issueHandlers>
-        <RedundantCondition errorLevel="suppress"/>
-        <RedundantCast errorLevel="suppress"/>
-    </issueHandlers>
-    <projectFiles>
-        <directory name="src" />
-    </projectFiles>
-</psalm>
diff --git a/src/Grammar/Repetition.php b/src/Grammar/Repetition.php
index 88be291..4032faf 100644
--- a/src/Grammar/Repetition.php
+++ b/src/Grammar/Repetition.php
@@ -45,8 +45,8 @@ public function __construct($rule, int $gte = 0, $lte = \INF)
         \assert($lte >= $gte, 'Min repetitions count must be greater or equal than max repetitions');
 
         $this->rule = $rule;
-        $this->gte  = $gte;
-        $this->lte  = \is_infinite($lte) ? \INF : (int) $lte;
+        $this->gte = $gte;
+        $this->lte = \is_infinite($lte) ? \INF : (int) $lte;
     }
 
     public function getTerminals(array $rules): iterable
diff --git a/src/Grammar/Terminal.php b/src/Grammar/Terminal.php
index 2dd4914..91a4d48 100644
--- a/src/Grammar/Terminal.php
+++ b/src/Grammar/Terminal.php
@@ -9,9 +9,9 @@ abstract class Terminal extends Rule implements TerminalInterface
     /**
      * @readonly Should not be modified in runtime.
      */
-    public bool $keep = true;
+    public bool $keep;
 
-    public function __construct(bool $keep)
+    public function __construct(bool $keep = true)
     {
         $this->keep = $keep;
     }
diff --git a/tests/Functional/GrammarGeneratorTest.php b/tests/Functional/GrammarGeneratorTest.php
index 0127fab..d3efb49 100644
--- a/tests/Functional/GrammarGeneratorTest.php
+++ b/tests/Functional/GrammarGeneratorTest.php
@@ -48,10 +48,10 @@ public function testHelpers(): void
         });
 
         $expected = [
-            0     => new Lexeme('digit'),
-            1     => new Lexeme('plus'),
-            2     => new Concatenation([1, 0]),
-            3     => new Repetition(2),
+            0 => new Lexeme('digit'),
+            1 => new Lexeme('plus'),
+            2 => new Concatenation([1, 0]),
+            3 => new Repetition(2),
             'sum' => new Concatenation([0, 3]),
         ];
 
diff --git a/tests/Functional/SimpleSumParserTest.php b/tests/Functional/SimpleSumParserTest.php
index 4085074..586d3a3 100644
--- a/tests/Functional/SimpleSumParserTest.php
+++ b/tests/Functional/SimpleSumParserTest.php
@@ -55,21 +55,21 @@ private function parseSum(string $expr): iterable
     {
         $lexer = new Lexer([
             'T_WHITESPACE' => '\s+',
-            'T_DIGIT'      => '\d+',
-            'T_PLUS'       => '\+',
+            'T_DIGIT' => '\d+',
+            'T_PLUS' => '\+',
         ], ['T_WHITESPACE']);
 
         $grammar = [
-            0        => new Lexeme('T_DIGIT'),
-            1        => new Lexeme('T_PLUS'),
-            2        => new Repetition('suffix'),
-            'sum'    => new Concatenation([0, 2]),
+            0 => new Lexeme('T_DIGIT'),
+            1 => new Lexeme('T_PLUS'),
+            2 => new Repetition('suffix'),
+            'sum' => new Concatenation([0, 2]),
             'suffix' => new Concatenation([1, 0]),
         ];
 
         $parser = new Parser($lexer, $grammar, [
             Parser::CONFIG_INITIAL_RULE => 'sum',
-            Parser::CONFIG_AST_BUILDER  => $this,
+            Parser::CONFIG_AST_BUILDER => $this,
         ]);
 
         return $parser->parse($expr);