Lua: how to call disaster()

[taka]

Hi. I'm using freeciv 2.5.7.

Lua reference manual says:

disaster(Disaster disaster, City city)

What is the Disaster?
I wrote the code to cause a disaster (fire) in event_city:

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local disaster_event = "disaster_fire"
disaster(disaster_event, event_city)

but I got a error.

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lua error: attempt to call global 'disaster' (a nil value)

what's wrong?

[taka]

Okey I understand.
disaster() function is used for event signal.