If a Runner uses Eureka! and reveals Angel Arena, can they set X to 10 and then use Eureka!'s effect to install Angel Arena with 10 power counters, without spending any credits? If you use Eureka! on Angel Arena, then the X value the Corporation chooses to pay is lowered by 10, so if you choose to pay 10 credits you actually pay 0 credits. The card sees that you payed zero credits, because X was lowered from 10 to 0, and you get 0 power counters.