
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
The polynomial $-2+x^{m-1}y+x^{m-3}-nx^{2}y^{m-3}$ is a fourth-degree trinomial in terms of $x$ and $y$.  
$(1)$ Find the values of $m$ and $n$;  
$(2)$ Rearrange this polynomial in descending powers of the variable $x$ and write down its constant term.

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