
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
In the Cartesian coordinate system, if line $l_1$ is given by the parametric equations
$$
\begin{cases}
x = 2s + 1 \\
y = s
\end{cases}
$$
(where $s$ is a parameter) and line $l_2$ is given by the parametric equations
$$
\begin{cases}
x = at \\
y = 2t - 1
\end{cases}
$$
(where $t$ is a parameter) are parallel, then the value of the constant $a$ is ______.

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