
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
If the values of two unknowns in a system of two linear equations are opposite to each other, we call this system of equations a "related system of equations."  
$(1)$ Determine whether the system of equations $\left\{\begin{array}{l}{3x-y=4}\\{x-3y=4}\end{array}\right.$ is a "related system of equations," and explain the reason;  
$(2)$ If the system of equations $\left\{\begin{array}{l}{x+3y=4-a}\\{x-y=3a}\end{array}\right.$ concerning $x$ and $y$ is a "related system of equations," find the value of $a$.

Present the answer in LaTex format: \boxed{Your answer}
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