
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
Let $a$ and $b$ be positive numbers, and consider the following propositions:  
① If $a^2 - b^2 = 1$, then $a - b < 1$;  
② If $\frac{1}{b} - \frac{1}{a} = 1$, then $a - b < 1$;  
③ If $e^a - e^b = 1$, then $a - b < 1$;  
④ If $\ln a - \ln b = 1$, then $a - b < 1$.  
Among these, the true propositions are \_\_\_\_\_\_.

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