
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
Among the following propositions, the number of correct propositions is _______  
① If vectors $\overrightarrow {a}$ and $\overrightarrow {b}$ are collinear, then the line containing $\overrightarrow {a}$ and $\overrightarrow {b}$ is parallel;  
② If the lines containing $\overrightarrow {a}$ and $\overrightarrow {b}$ are skew, then $\overrightarrow {a}$ and $\overrightarrow {b}$ are definitely not coplanar;  
③ If vectors $\overrightarrow {a}$, $\overrightarrow {b}$, and $\overrightarrow {c}$ are pairwise coplanar, then $\overrightarrow {a}$, $\overrightarrow {b}$, and $\overrightarrow {c}$ must also be coplanar;  
④ Given three vectors $\overrightarrow {a}$, $\overrightarrow {b}$, and $\overrightarrow {c}$, any vector $\overrightarrow {p}$ in space can always be uniquely expressed as $\overrightarrow {p} = x\overrightarrow {a} + y\overrightarrow {b} + z\overrightarrow {c}$.

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