
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 true one is __________  
 (1) In a plane, the locus of points whose sum of distances from two fixed points $F_{1}$ and $F_{2}$ is a constant is an ellipse;  
 (2) If vectors $\overrightarrow{e_{1}}$, $\overrightarrow{e_{2}}$, $\overrightarrow{e_{3}}$ are three non-collinear vectors, and $\overrightarrow{a}$ is any vector in space, then there exists a unique set of real numbers $\lambda_{1}$, $\lambda_{2}$, $\lambda_{3}$ such that $\overrightarrow{a}=\lambda_{1} \overrightarrow{e_{1}}+\lambda_{2} \overrightarrow{e_{2}}+\lambda_{3} \overrightarrow{e_{3}}$;  
(3) If proposition $p$ is a sufficient but not necessary condition for proposition $q$, then $\neg p$ is a necessary but not sufficient condition for $\neg q$.

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