IRequirement $1$:

Be a silent process $($no exceptions$).$ Do not output any logs$.$ Do not raise any exceptions. Your program should never return any output to the user.

Requirement $2$:

A valid file must start with the character $\text{'}$S$\text{'}$ and end with the character $\text{'}$E$\text{'}.$ If any other characters $($including lowercase $\text{'}$s$\text{'},\text{'}$e$\text{'},$ etc.$)$ are the first or last characters, then the file is considered invalid.

Requirement $3$:

Applications use ABopenfile$()$ to create or open a file. Files are created by setting create$=$True when calling ABopenfile$().$ The reference monitor will create a valid file called filename.a and an empty file we will write to$,$ called filename.b$.[$If you are the attacker, think about how you can mess with the create value to break the monitor.$]$

Requirement $4$:

When close$()$ is called on the file, if both filename.a and filename.b are valid, the original file's data is replaced with the data of filename.b$.$ If filename.b is not valid, no changes are made.

Requirement $5$:

When writing, your reference monitor should not allow the data to begin with a space $().$