
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
Each New Year, starting from the first year AD, Methuselah, who is still alive today, sends a greeting to his best friend. The greeting formula remains unchanged for almost two millennia: "Happy New Year 1", "Happy New Year 2", "Happy New Year 3", and so on, "Happy New Year 1978", and finally, "Happy New Year 1979".

Which digit has Methuselah used the least frequently so far?

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