
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 an ellipse $\dfrac {x^{2}}{a^{2}}+ \dfrac {y^{2}}{b^{2}}=1\;(a > b > 0)$, lines $l_{1}: y=- \dfrac {1}{2}x$, and $l_{2}: y= \dfrac {1}{2}x$. Let $P$ be any point on the ellipse. A line parallel to $l_{1}$ passing through $P$ intersects $l_{2}$ at point $M$, and a line parallel to $l_{2}$ passing through $P$ intersects $l_{1}$ at point $N$. If $|PM|^{2}+|PN|^{2}$ is a constant, then the eccentricity of the ellipse is \_\_\_\_\_\_.

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