Ensure seekBack is defined

lib/Encoding/AbstractEncoding.php

@@ -19,6 +19,8 @@ abstract class AbstractEncoding implements Encoding {
 
     public $posErr = 0;
 
+    abstract protected function seekBack(int $distance): int;
+
     public function __construct(string $string, bool $fatal = false, bool $allowSurrogates = false) {
         $this->string = $string;
         $this->lenByte = strlen($string);

lib/Encoding/SingleByteEncoding.php

@@ -70,6 +70,11 @@ abstract class SingleByteEncoding extends AbstractEncoding implements StatelessE
         }
     }
 
+    protected function seekBack(int $distance): int {
+        // stub: not used
+        return 0;
+    }
+
     public function posByte(): int {
         return $this->posChar;
     }

lib/Encoding/XUserDefined.php

@@ -77,6 +77,11 @@ class XUserDefined extends AbstractEncoding implements Encoding {
         }
     }
 
+    protected function seekBack(int $distance): int {
+        // stub: not used
+        return 0;
+    }
+
     /** Returns the current byte position of the decoder */
     public function posByte(): int {
         return $this->posChar;