Why can't I transform tundra?

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I'm playing freeciv 2.5 with ruleset civ2civ3. I've Engineer on Tundra tile, and want to transform it into Plains, but cannot: proper action is in the menu, but is grayed out. Why? Is this a bug or am I missing something?
It's linux version, under Ubuntu 14.04. Tried under gtk2 and sdl client, the same result.
Best Answer
Ok, I've found the answer in the docs. Fusion Power technology must be known to make such transformation possible.
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