Prepare for block level program handling

src/Automata/Storage/ProgramMemory.php

@@ -13,9 +13,7 @@ class ProgramMemory
 
 	public function save(Program $program): void
 	{
-		if ($program->N > 9998 || $program->N < 9001) {
-			throw new InvalidArgumentException('Program number must be in range 9001 - 9998');
-		}
+		$this->guardInvalidProgramNumber($program->N);
 		$this->memory[$program->N] = $program;
 	}
 
@@ -24,4 +22,13 @@ class ProgramMemory
 	{
 		return @$this->memory[$N];
 	}
+
+
+	private function guardInvalidProgramNumber(int $N): void
+	{
+		if ($N > 9998 || $N < 9001) {
+			throw new InvalidArgumentException('Program number must be in range 9001 - 9998');
+		}
+	}
+
 }