error: 'Queue' does not name a type

I encountered the following error message when trying to use the Queue.h library in the Arduino IDE coding for a ESP-32 developer board: `error: 'Queue' does not name a type`

The fix was to change 
```
#ifndef QUEUE_H
#define QUEUE_H
```
to
```
#ifndef Queue_h
#define Queue_h
```
in the Queue.h file.