In general prerequisites are not absolutely required to get an advance, but without the prereq. For instance Drainage allows more agricultural sites because the civ can now drain swamps. As you work down the document things get increasingly more sketchy. Most items don't have game functions associated with them yet, but will in the next version. I've run out of gas temporarily on this, but will get back to it.
Suggestions are welcome.
PreRequisite 1 . . Simple Machines . Met2 - Iron Working . The Automobile Sci13 Contemporary Chemistry . PreRequisite 2 . . . .