
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
Given the following four propositions:

① Two lines that do not have common points are parallel;

② Two lines that are perpendicular to each other intersect;

③ Lines that are neither parallel nor intersecting are skew lines;

④ Two lines that are not in the same plane are skew lines.

Among these, the correct propositions are __________. (Fill in the serial numbers)

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