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RESOURCE_EXTRACTOR
an oil derrick
they can be built on oil resources
power gens are needed to make oil derricks pump oil - see POWER_GEN for more info on that
because players can usually only have max 5 power gens, that translates in to a max of 20 derricks that you can "power" in most cases. if power gen limit is different, then obviously max usable derricks is differenet. usable derricks = power gens * 4
see also derrickPositions[] (which is actually positions of available oil resources)