
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 a set $M$ consisting of all functions $f(x)$ that satisfy the following conditions: (1) When $x \in [0, +\infty)$, the function value is a non-negative real number; (2) For any $s, t$, it holds that $f(s) + f(t) \leq f(s+t)$. Among the three functions $f_1(x) = x$, $f_2(x) = 2^x - 1$, $f_3(x) = \ln(x+1)$, the ones that belong to set $M$ are.

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