
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
Let $a$ and $b$ be two non-coincident lines, and let $\alpha$ and $\beta$ be two non-coincident planes. Consider the following propositions:
1. If $a \parallel \alpha$ and $b \parallel \alpha$, then $a \parallel b$.
2. If $a \perp \alpha$ and $b \perp \alpha$, then $a \parallel b$.
3. If $a \parallel \alpha$ and $a \parallel \beta$, then $\alpha \parallel \beta$.
4. If $a \perp \alpha$ and $a \perp \beta$, then $\alpha \parallel \beta$.

The true propositions are __________. (Fill in the sequence numbers)

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