Official rule text
90.14 When researching technology, a player can exhaust a planet prerequisites. they control that has a technology specialty to ignore one c There are four colors of technologies as follows: prerequisite symbol of the matching type on the technology card they are researching.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 90.14