Core memory involved a destructive read operation where to determine whether a core stored a $0$ or a $1,$ it had to be set to $0.$ The required then that a read be immediately followed by a write to restore the value that was stored there. This led to what was known as the read$-$modify$-$write memory cycle. It was used for various purposes, such as being able to increment a variable in memory in the same time that it could be read. Describe how this read$-$modify$-$write cycle made a test$-$and$-$set instruction natural to implement.