Alignment with other buffer types

src/DotNext.Tests/Buffers/BufferWriterTests.cs

@@ -324,4 +324,21 @@ public static void ChangeWrittenCount()
         buffer.WrittenCount = 1;
         Equal(42, buffer[0]);
     }
+
+    [Fact]
+    public static void AdvanceRewind()
+    {
+        var buffer = new PooledArrayBufferWriter<int>();
+
+        Throws<ArgumentOutOfRangeException>(() => buffer.Rewind(1));
+
+        buffer.Add(42);
+        Equal(1, buffer.WrittenCount);
+
+        buffer.Rewind(1);
+        Equal(0, buffer.WrittenCount);
+
+        buffer.Advance(1);
+        Equal(42, buffer[0]);
+    }
 }

src/DotNext/Buffers/BufferWriter.cs

@@ -242,6 +242,20 @@ public void Advance(int count)
         position = newPosition;
     }
 
+    /// <summary>
+    /// Moves the writer back the specified number of items.
+    /// </summary>
+    /// <param name="count">The number of items.</param>
+    /// <exception cref="ArgumentOutOfRangeException"><paramref name="count"/> is less than zero or greater than <see cref="WrittenCount"/>.</exception>
+    public void Rewind(int count)
+    {
+        ThrowIfDisposed();
+        if ((uint)count > (uint)position)
+            throw new ArgumentOutOfRangeException(nameof(count));
+
+        position -= count;
+    }
+
     /// <summary>
     /// Returns the memory to write to that is at least the requested size.
     /// </summary>