diff --git a/src/Argument/ArgumentValueList.php b/src/Argument/ArgumentValueList.php
index 524d545..814389f 100644
--- a/src/Argument/ArgumentValueList.php
+++ b/src/Argument/ArgumentValueList.php
@@ -4,11 +4,15 @@
 class ArgumentValueList {
 	protected $valueMap = [];
 
-	public function set(string $key, string $value):void {
+	public function set(string $key, string $value = null):void {
 		$this->valueMap[$key] = $value;
 	}
 
 	public function get(string $key):string {
 		return $this->valueMap[$key];
 	}
+
+	public function contains(string $key):bool {
+		return isset($this->valueMap[$key]);
+	}
 }
\ No newline at end of file
diff --git a/src/Command/Command.php b/src/Command/Command.php
index b4fedd4..04ad96f 100644
--- a/src/Command/Command.php
+++ b/src/Command/Command.php
@@ -28,7 +28,7 @@ abstract class Command {
 	/** @var Parameter[] */
 	protected $requiredParameterList = [];
 
-	public function setOutput(Stream $output) {
+	public function setOutput(Stream $output = null) {
 		$this->output = $output;
 	}